There are different ways of altering timing.
Usually, an esc with boost will allow a base/primary advance timing that is active throughout the entire power range, and a secondary advance timing (higher than the primary) to engage at a user definable rpm (in steps).
The reason for having two timing parameters is to increase efficiency.

In an esc with only one timing parameter (no boost):
Having too much timing throughout the entire powerband will cause excessive heat, higher amp draw, and lower runtimes (while yielding more power up top). Too little and you are losing out on top end speed.

Having two timings (boost) allows the efficiency of lower timing at lower rpms to keep amp draw and heat down, therefore longer runtime, and the higher timing to produce more power up top. So basically a 'best of both worlds' scenario.