Push Block (also known as Advancing Guard in Marvel vs Capcom 3) is a defensive mechanic that was originally introduced in Marvel vs Capcom 2. It also appears in Skullgirls and Injustice: Gods Among Us.
Executed by pressing while blocking an attack, the attacking player is pushed away. This can be used to interrupt block strings, giving the player some breathing room to retaliate. Because of the distance the opponent is pushed back, this mechanic is often used to escape from overwhelming offensive pressure and swing momentum of a match. In most games where Push Blocking exists that also include an Assist mechanic, the assist characters will be immune to Push Blocking. Another limitation of Push Blocking is that the character executing the mechanic will be put into a delay where they are unable to perform any action other than blocking.
In M.U.G.E.N, Push Blocking is usually accomplished utilizing an invincible helper with a large, vertical hurtbox in conjunction with the PlayerPush state controller, along with some horizontal velocity, in order to simulate the push back effect of the mechanic.