Illegal crackback

I was just in the rulesbook and referee signals ssection of this website and saw the signal for an illegal crackback. can any body tell me what an illegal crackback is?

That's a very good question, because I've often been confused by that myself. Here's my understanding of it, and if I'm wrong hopefully someone can explain it to me.
When a player is being blocked, it is illegal for someone else to block that same player below the waist.
Example: say a defensive lineman is rushing the QB, and an o-lineman is blocking him. If another offensive lineman dives at the back of the d-lineman's legs, that's a crackback block. I believe it's a 15 yard penalty.
That's how I've always interpreted it. If someone knows otherwise, please fill us all in.

thanks again BigDave. your awsome!

I'm Back!!!! Well actually it's my first time posting on the new forum. Hi Y'all.

To answer the question, an illegal crackback is when a player starts 3 yards outside the tackle and blocks another player below the waist to the inside. For example, a WR lined up outside comes in to the middle of the field on an outside run play to his side, and takes out the knees of the pursuing LB. It is a 15 yard penalty.

BigDave, you are thinking about a chop block, which is also a 15 yard penalty.

Yup, you're right. After I logged off I realized that I was thinking about chop blocking.

Sorry if I confused anyone. Thanks AATW.

welcome back alsalltheway