Its all about traction managment. You tires only have a finite amout of traction which is modified by the amout of weight/load they are under. Trail Braking is a technique where you slowly shift the avalable grip from braking to turning. This allows for faster corner entry and deeper braking. The side effect is that you can also hold more load on the frot tires to aid in cornering grip. There are some problems if you don't manage the avalable grip though. Entering too fast can cause you to understeer from two different conditions. Either over loading the front tires or under loading them. Its a tough ballancing act, but getting it right can make a big difference in lap times. Get it wrong and you lap times will be even slower then a simple slow-in fast-out cornering technique.
Not all corners are approate for this type of technique. Constant radius corners and decreasing radius corners ar the best place to apply it, but increasing radius corners work better with a slow-in fast-out technique combined with a late entry late apex line. You also need to look at what leads in to the corner and what is beyond the corner. For example if you have a long straight leading in to a tight section its is benifitial to carry as much speed as deep in to the corner as you can. On the other hand if its a corner exiting a slow section and leading on to a straight then maximixing exiting speed is key. So look at the track, look at what leads in to and whats beyond the corner, look at the corners geomitry and apply the more approate technique to each corner and corner complex.
