In trail braking, you decrease brake pressure while increasing steering lock. During this brake decrease and lock increase, your radius will decrease. Think of the exit as the same thing in reverse, decreasing lock and adding throttle. You would never trail brake in a constant radius, because holding radius while braking (thus decreasing your speed) means you're just slowing down and using less than maximum lateral grip. After all you entered that radius at a high speed - why go slower in the middle? Unless you messed up your line, that is.
If you had a hairpin, there would be an angle, x degrees, which would allow you a constant radius, that would allow you to use up all the road on entry and on exit, and hit the apex, in one sweeping constant radius arc. You could even draw that arc on paper if you had a track map, with a compass.
Naturally, because in trailing the entries and exits you are entering the corner at less than x degrees of lock, and exiting at the very end at less than x degrees of lock, geometrically speaking, if you trail the entries and exits, at the apex itself you would hold more lock to compensate, and hold slightly less apex speed. If you were to look at it from above, the line would look like you took the straighter entry line of an early apex, with the straighter exit line of a late apex, sacrificing raw apex speed to do it.
Most F1 drivers tend to trail brake right up to the apex. Look for onboards with telemetry.
Take a look at Webber in Brazil:
[youtube]
http://www.youtube.com/watch?v=NHgoij63Ae0[/youtube]
In the more prominent braking zones, such as Turn 1 and 4; and when Mark was very clearly trail braking; the G-meter reads constant the whole way; basically having the same net acceleration, just that it shifts in direction from the car's longitudinal axis, onto the lateral, by decreasing the braking in harmony with increasing the steering.
EDIT: Saw that FOM has taken the video down. Here's the best I could find so far:
[youtube]
http://www.youtube.com/watch?v=Y_BUIEhvmPg[/youtube]
Same thing - look at turns such as Turn 1. The acceleration (measured in G) is pretty much static throughout the corner, allowing for having less downforce as the car slows, but look at how they basically are shifting the red dot clockwise or counterclockwise on at a constant distance to the center of the accelerometer.
