F1technical.net

I’m pretty sure the driver’s initial effort is more of a kick than a push. The objective is to get the brake application as fast as possible to take advantage of the traction at high downforce levels. The additional inertial load from the drivers leg mass no doubt helps here.

I seem to remember that at the time of the McLaren spygate episode one of the projects that was revealed was aimed at speeding up the initial brake application by technical means.