Trail-braking is a subtle driving technique that allows for later braking and increased corner entry speed. The classical technique is to complete braking before turn-in. This is a safer, easier technique for the driver because it separates traction management into two phases, braking and cornering, so the driver doesn't have to chew gum and walk at the same time, as it were. With the trail-braking technique, the driver carries braking into the corner, gradually trailing off the brakes while winding in the steering. Since braking continues in the corner, it's possible to delay its onset in the preceding straight braking zone. Since it eliminates the sub-optimal moments between the ramp-down from braking and the ramp-up to limit cornering by overlapping them, entry speeds can be higher. The combination of these two effects means that the advantage of later braking is carried through the first part of the corner. In many ways, this is the flip side to corner exit, where any speed advantage due to superior technique gets carried all the way down the ensuing straight.

The magnitude of the trail-braking effect is much smaller than a great corner exit, perhaps a car length or two for a typical corner. Done consistently, though, it can accumulate a whole second or more over a course.

When students are taught to drive as Novices, I believe they should be braking straight and then turning in for a variety of reasons such as:

It's a small effect compared to the big-picture basics, like carrying speed out of a corner, that everyone must learn early on. It's difficult to learn and Novices need to learn and master more basic skills first.
Mistakes with it are ugly.

As you progress in this sport, the general level of driving skill will increase to the point where it's no longer optional, unless you're content with not gaining additional speed. As with most driving skills, it's difficult to get a feel for the limits without exceeding them from time to time. However, exceeding the limits at trail braking has some of the worst consequences one can invite on a race track, typically worse than those from mistakes at corner exit. It's definitely a big risk for a small effect, justified only because it accumulates. Blowing it results in too high an entry speed. You get:

Inappropriate angular attitude in the corner
Immediate probing of the understeer or oversteer characteristics of the car
Surprise, pop quiz on the driver's car-control skills
Missed apex and track-out points
A looming penalty cone, gravel trap, tire barrier, concrete wall, tree, etc.
When you bounce back from that impact, you can hit other cars, spectators, corner workers, etc.
Anything else that can go wrong in a blown corner

That's one of the reasons you may not target this skill for your personal driver-development work. It's hard to do at all and harder to do it consistently and may not seem worth it.



So why take the risk of learning to trail brake?

Go wheel-to-wheel on the track with equal cars, and the issue becomes instantly and visually obvious. You may be just as fast in the corner, coming out of the corner, down the straight. You may have perfect threshold braking. You may have perfect turn-in, apex and track out points. But that little extra later braking and entry speed will allow the trail-braker to take away several feet every corner. Corner after corner, lap after lap, he will gobble you up.

For those of us that want to push our skills to the limit, trail braking should be on your agenda of skills to perfect. Don't try to master it in one weekend or even one season, this is one of those things that must be crept up on and done so in a manner that keeps you safe. You must develop the needed confidence in yourself and your car in order to achieve the results you want. It is NOT for the Novice driver.

In order to be properly performed, the driver must have excellent sense of the vehicle's behavior 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.