How does the stick pusher prevent an aircraft from stalling?
A simple explanation is that a stick pusher is a:


  ... device that pushes forward on the elevator control system
  whenever the aircraft’s angle of attack reaches the pre-determined
  value, and then ceases to push when the angle of attack falls
  sufficiently.


A detailed explanation is:


  A stick pusher system incorporates both a passive monitoring component
  and an active intervention component that consists of an hydraulic or
  electro-mechanical pusher mechanism. The monitoring component
  constantly evaluates critical flight parameters such as airspeed, load
  factor and flap setting against the aircraft angle of attack. If the
  angle of attack reaches a predetermined value, the intervention
  component of the system is activated and pushes forward on the
  elevator control system until the angle of attack is reduced to an
  acceptable value. The system operates autonomously and without any
  required action by the pilots.


FAA has an advisory circular (AC) for stick pusher training.

For Embraer 145, this video shows a demo of stick shaker, stick pusher and the warning systems.