Crackback block

[JugglingReferee]

Originally posted by lawref
Can anyone give me a definition of a crackback block and when one would most likely occur?

Canadian Amateur Rules:

The crackback seems to be similar in Canada and in the US.

The criteria in the Great White North are:
(1) the block must occur in an area 5 yards ahead of the LS and all the way back to Team A's dead ball line.
(2) it must be below the waist
(3) it must towards the ball at the snap of the ball
(4) the person flagged for the block must have been, at some point before the snap, more than 3 yards outside the widest interior lineman

Penalty: UR. Signal: UR, then slap front of thigh with palm.