Originally Posted by Mr teX
the bolded only applies to defensive lineman in the trenches where room is limited. If a DL-man is actively engaged with an o-lineman up high, no other OL-man can intentionally hit said DL-man low at the same time; this prevents guys from getting rolled up on or getting their knees torn up.
It still happens on occasion accidentally (allegedly) but there is a rule in place to prevent it from becoming part of strategy.
everyone else out in the field where space is plentiful are fair game however.
/Tripping: In most circumstances, players are not allowed to tackle below the knees. A chop block occurs when two players tackle an opponent, with one going high and the other hitting low.
Tripping can be called when a player on the ground reaches out with his feet or hands to bring down an opponent who does not have posession of the ball.
I think it's dangerous, especially when a defender undercuts from behind.
Not good for your ankle.