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

NCAA
 

The "crackback" refers to an offensive player who is usually spread out away from the main body of the formation and who runs back in towards the ball at the snap in hopes of blocking a linebacker or defensive backer who is moving towards the blocker but looking into the offensive backfield at the ball carrier such that he does not see the block coming.

If the block is above the waist and not from the back, it is legal. If it is below the waist and the force of the block is back in towards the original position of the ball at the snap, it is illegal.

The block usually occurs on sweeps or option plays to the outside.

We were just talking about these at our meeting tonight. I agree with TXMike's analysis. Only thing I would add as a "read" on who might crackback: if a wide player is in motion toward the ball before the snap, he is a likely candidate to crackback. As already mentioned, however, its only a foul if the block is below the waist, a block in the back, or a clip.

Another thing to add is defensive teams pick up on this fast. Watch the defensive end or LB as he will watch for the "crackback" block and will intentionally turn to get blocked in the back as the last second. On that I have no flag and if he or the coach complains, tell him why.

Complete agreement with both of you. Those guys in motion towards the ball are very good candidates to foul. And re turning the back, at first I was going to say it is almost impossible to have a block in the back on the crackback situation because the defenders are usualy square with the LOS or running straight at the impending blocker. But as I thought about it more I remembered plays where the "crackback" guy did end up in a position where he was behind the defender and then blocked him in the back, towards the LOS.

Another hint
 

A hint that I got from a college official on calling blocks in the back; watch the action of the player that was blocked after the contact. If he falls forward on his numbers, then he probably was blocked in the back. If he falls sideways, then the blocker will probably get the benefit of the doubt. This is especially helpful on kicking plays.

There is no absolute, and there is no substitute for seeing the entire play. Just a tip that might help.

"Crackback" is a coaching term and is not part of the NF rule book. As many have stated earlier, as long as the block is legal, you have nothing.

The word crackback was mentioned at least three times at our state rules meeting and at least that many times at my first association meeting.

It's not in the rulebook, but when a coach is screaming from the sidelines to "watch the crackbacks" you'd better know what the heck he's talking about. If it's in the back or below the waist it is a foul and one that needs to be seen.

Rich

Rich, We all agree on that but don’t get sucked in by the coaches that this act of a motion man blocking back in towards the LB or end, what they call a “crackback” is illegal, that’s football. We watch for legal blocks.

We don't disagree. We need to make sure we see the block so we know whether it is legal or warrants a flag.

Lots of guys miss that block because they are watching the guy carry the ball.

Rich

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.