The ability of tanks to move through unoccupied enemy territories, capturing them along the way. AAG40 gives mechanized units this ability, if they are moving with tanks.
There is a common rules question with blitzing. Non-combat units such as factories and AA guns do stop blitzing.
Blitzing can play an important role in A&A. The need to stop blitzing is a major reason to leave at least 1 infantry on the front line. When different players take their turn before a response, a strafing attack can sometimes be used to clear a screening unit and allow a blitz.