When on foot, only the overhead swing can result in a crushing blow. It tends to occur against a shield in most cases, and very rarely against weapons. It depends on your speed bonus, the resistance of the shield, and some luck. With this effect, a weapon can literally break through blocks, dealing damage and stuns over a shield or weapon block.
Can crush through blocks is special effect only some kinds of Melee weapons can deal in Warband and With Fire & Sword.