Is it illegal for the tight-end, off the whistle, to lung head first at the defensive-end and hit/block him below the knees?

I'm not exactly sure what "off the whistle means," but I'll assume you mean immediately after the snap. For NF rules, if both players are on their LOS and in the Free Blocking Zone (FBZ) at the snap and the contact occurs in the FBZ, then the block below the waist is legal. Generally the TE is in the FBZ at the snap unless the offense is using larger than average splits between their O-linemen.

And in case you don't know, the FBZ is a retangular area extending laterally 4 yards either side of the spot of the snap and 3 yards behind each line of scrimmage. The zone disintegrates when the ball leaves the zone (i.e. when the ball leaves the 8 x 6 zone, players can no longer legally block in the back, block below the waist, or clip); this last part is especially important, because if the team is in shot-gun formation, the ball will only be in the zone for a split second after the snap.

"off the whistle?"
 

Without further information and not knowing what "off the whistle" means, it can be. If the TE does not fit the requirements of the "Free-Blocking zone," then the answer is yes it is illegal. But honestly, we need more information or discription of your situation to know for sure.

Peace

The key: Is the TE in the FBZ? It varies according to splits.

clarification
 

Sorry for the confusion! I meant off of the snap. The DE is in the FBZ. At the snap the TE shoots at the DE shin area (below the knees). The following website states ILLEGAL LOW BLOCK: A player can not block a person below the knees.

http://www.football.com/penalties/fi...#Illegal%20low

Re: clarification
 

REPLY: The website you directed us to must be authored by TV football commentators (Theismann, Madden, etc.). Just read the definition of "chop block" and you'll realize that site is for the birds! By the way, an illegal low block is one that's below the <u>waist</u>, not below the knees. And such blocks can be legal if they meet the criteria that the other posters have mentioned.

Well that is true. There are illegal low blocks and on most areas of the field any block initiated below the waist is illegal. However while in the FBZ there are instances where blocking below the waist is legal.

First for blocking below the waist when both players are on the LOS and in the FBZ and the contact is in the zone.

Second for clipping by the offense against the defense when both players are on the LOS and in the FBZ and the contact is in the zone.

But not legal if it is a delayed block by an offensive player against a defensive player already engaged by another offensive blocker.

Another case where it is legal to first contact a player below the waist with no resrictions is when the player being blocked has no feet on the ground (normally while jumping to block a pass or kick).

Does the NCAA use the same FBZ?

No.

They have different perameters to block below the waist. They do have a zone, but it is acceptable if certain things are taking place. This year they made it much easier than it used to be, but it is not the same.

Peace

If the line is in a standards T, G, C, G, T, TE with the TE in the line the TE is considered in the free blocking zone and can block low. If the TE is on the "strong" side of an unbalanced line then he is not in the FBZ and cannot block low. As well if they run a double TE with both TE's on the same side, the inside TE can block low but the outside one cannot..
Remember the defender he hits low must be in the FBZ and must be on (no more than 1 yard off) the LOS. So if the TE that can block below the waist hits a LB off his outside shoulder then he should be flagged for the block below the waist.

The reason I asked is that I saw a game saturday where the OT got beat on a screen pass and after DE was past him he dove into the back of DE's legs. I was always under the impression that pretty much anything goes as long as the initial contact was in front and contact is maintained but once you lose contact you can no longer block in the back. Am I wrong? Speaking NCAA.

That to me sounds like a clipping foul. 15 yards.