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Trail braking is a motorcycle riding and driving technique where the brakes are used beyond the entrance to a turn and are gradually released up to the point of apex.
In a broader scope, trailing off the braking pressure either while straight line braking or, as above, after turn in has begun, allows for a less abrupt and more accurate final corner entry speed adjustment. Some corner entries, such as decreasing radius turns, are more adapted to the leaned over trail braking technique. In turns where a quicker steering action is more applicable, trailing the brake while turning in is unnecessary.

Usage In Motorcycling
In applying this technique, motorcycle riders approach turns applying both front and rear brakes to reduce speed. As they enter the turn, they slowly ease off the brakes, gradually decreasing or "trailing" off the brakes as motorcycle lean increases. This is done for several reasons. First, this gives more traction because the front tyre is forced into the pavement under the weight of the vehicle. Second, as the brakes are applied and the weight shifts forward the forks are compressed. The compression of the forks changes the motorcycles steering geometry, decreasing stability in a way that makes the motorcycle want to lean and change direction. Thus, trail braking can be used as method used to help the motorcycle change direction.
Additionally, decreasing speed decreases the motorcycles cornering radius, while, conversely, accelerating while turning increases the motorcycles cornering radius. Traditionally, trail braking is done exclusively with the front brake even though trailing the rear brake will effectively slow the motorcycle, also decreasing the turning radius.
It should be noted that the rider's ability to correctly choose his turn in, apex and exit points reduces or eliminates the necessity of prolonged trailing of the brakes into turns.
Finally, trailing off the brakes while entering blind or tight corners allows the rider to slow if something unexpected blocks the rider's path. Because the motorcycle is already on the brakes and the front tyre is getting additional traction from already slowing, the rider can slow even more with very little risk, depending on surface conditions. However, applying the brakes after the motorcycle is already leaned over can be exceedingly risky depending on surface conditions and lean angle. There is already roughly 70% of the bike's weight forward from being off the gas so applying braking pressure significantly increases that load on the smaller front tyre.
This technique is commonly used when racing, but can enhance control and add more evasive options for street riders, making it very worthwhile to learn or at least understand.
Worth noting also is that most current (2000 through 2008) racing crashes happen while trailing the brakes into turns, amongst world class Moto GP riders.
Reverse Trail Braking is a variation of trail braking that can actually assist a motorcycle rider in turning his machine quicker in closely linked S curves or chicanes. In this use the rider applies some front brake as the steering change is being done. Because a motorcycle tends to "stand up" from the front brake application it more rapidly and efficiently brings the machine over to the opposite side, when used in conjunction with standard counter-steering inputs. This is an advanced riding technique which is only applicable to track riding and racing.

Risks In Motorcycling
There is a high degree of inherent risk with trail braking. Excessive use of the front brake can result in a loss of grip as the tyre's adhesion is split between braking and cornering forces. Effective trail braking requires a high degree of finesse from the motorcycle operator and has a very steep learning curve for riders wishing to learn and adopt this technique.
As motorcycles have a pronounced tendency to "stand up" when brakes are applied in a turn, the rider must initially counter that force with corresponding steering inputs. The handle bar pressure required, at this point, to hold the bike in the turn is one of the classic contributing causes of trail braking crashes.

Controversy In Motorcycling
Guides such as the Motorcycle Safety Foundation Basic Rider Course teach that the safest way for a beginning rider to approach a corner on a motorcycle is by performing all of the slowing before the entrance of the turn, discouraging the use of any brakes while the motorcycle is leaned over. Opponents of trail braking claim that because of the steep learning curve trail braking is or should be an exclusively race track or racing technique. However, proponents of trail braking believe that knowing and understanding how to slow while entering a corner gives the rider a greater safety margin, particularly in blind, decreasing radius or downhill corners.
Prominent motorcycle authorities disagree on the effectiveness of trail braking. Keith Code, founder of the California Superbike School teaches that the only way to effectively and accurately direct and turn a motorcycle is with counter steering, even going so far as to say in his 1983 book A Twist of the Wrist that "To tighten a turn and increase your lean angle, you must counter steer again". However, Freddie Spencer, founder of Freddie Spencer's High Performance Riding School as well as Nick Ienatsch, author of the 2003 book Sport Riding Techniques and chief instructor of Yamaha Champions Riding School, both teach that trail braking should be used in nearly every corner as a means to help the motorcycle change direction, advocating that trail braking gives the rider more control and significantly increases rider safety.
Additionally, Spencer and Ienatsh's programs advocate that the slower a motorcycle is going, the tighter the radius of the corner it can navigate. By using the brakes to slow the motorcycle down, the rider is effectively changing the motorcycle's direction by decreasing the radius of the corner. Conversely, applying throttle will increase the motorcycle's speed and effectively increase the radius of the corner. This is opposed to Code's teaching that only counter steering can affect the motorcycle's direction.
Eight times World champion, Valentino Rossi, has been quoted to say that trail braking is a racing technique and not a street technique. Rossi, unlike most world class racers, spent years riding on the streets and roads of Italy.

Usage In Four Wheel Vehicles
In 4-wheel vehicles trail braking pertains to using the brakes past the corner entrance (as opposed to the normally taught practice of releasing the brakes before starting the turn). This practice is used for creating weight transfer towards the front tyres, thus increasing their traction and reducing understeer. It works best in light vehicles that have their brake bias to the front.
In order to be properly performed, the driver must have excellent sense of the vehicle's behaviour and be able to keep the braking effort within very tight limits. Excessive braking effort may result in the vehicle heavily understeering, or - if the brake bias is set to nearly neutral - in the rear wheels locking, effectively causing the vehicle to spin as in a handbrake turn.
Once a driver has mastered trail braking, it can help enter the corners at higher speeds, or avoid an accident if the driver has entered a corner at a speed exceeding the vehicle's (or driver's) capabilities.

Usage In Racing
A drift-inducing technique called "the brake drift" is used in racing, involving a series of light trail-braking pulses (usually 2 or 3), followed by a momentary full-force braking and sharp releasing of the brakes. Mastering continuous trail braking as used under road conditions is a prerequisite for learning brake drifting. This is one of the most used drifting techniques in rally racing because - if done properly - allows the driver to enter and exit the corner with full throttle.
Depending upon cornering situations, techniques like trail braking can be used to maintain more speed upon entry of a corner, and attaining more grip while turning into the corner, and has an effect on apex selection. In this technique, brake pressure is applied slightly later than usual upon deceleration, and is maintained during steering input, sometimes all the way to the apex. The action of braking causes a weight transfer in the vehicle, shifting more weight from the rear of the car forward to the front tyres, increasing the normal force on them and in turn increasing the amount of traction the front (steering) wheels have. Because of the characteristics of weight transfer, this technique causes weight to be shifted away from the rear of the car, resulting in lower rear traction, and can be used to induce oversteer in some cases.