The following are rules that apply specifically to defense.
Probably the most consistently argued about at an OFL game is the tag. This is why a legal tag is important.
Tags must be two hands to the body. Not fingertips; not to a piece of shirt or the ball. Two hands to the body. Arms do not count.
The defender -- once he's made the tag -- should then loudly say "I Got Ya!" or "Down!" or something. Yelling "I Got Ya!" before actually tagging a ball-carrier is not cool (yes, it happens a lot..) The play stops when you announce "Got Ya!" or "Down!" Therefore if you say it before you actually get the ball-carrier, you're cheating. Illegal (phantom) tagging is a 15-pace penalty from the spot of the foul.
A offensive player is considered "down" when a defensive player tags with ball-carrier with two hands fully contacting the body (one hand if the player is on the ground). Touching only clothing (i.e. a small bit of T-shirt or the scrimmage vest), or tagging with fingertips, does not constitute a successful tag. Additionally, the ball does not count as part of the body.
Grabbing a ball carrier's clothes to stop him/her is a penalty. Tackling is not allowed.
Notes:
As such, defensive players should be very sure they have applied a legal tag before yelling "Gotcha". A false tag is a penalty - it stops play dead, and everyone gets mad and the whole thing generally sucks.
Other note: If the quarterback is directly under center (for a direct snap), defenders cannot reach across the line of scrimmage for a tag, even on a blitz. The defender must have his feet across the LOS before a tag is allowed.
The defense is responsible for collecting the ball and the scrimmage marker at the proper spot after the ball carrier has been downed. Purposely delaying this action during a "hurry-up" offensive series will be a delay of game penalty. An un-sportsmanlike conduct penalty will be assessed if the spotting is continually delayed.
Defenses count to "Four Mississippi" before crossing the line of scrimmage - unless:
In any of these instances the defense can cross the line of scrimmage immediately.
Note: "Crossing the Line of Scimmage" means any part of the body going over the line. Therefore reaching over the line to tag the QB before the "4 Mississippi" count is not allowed.
Defenses do not have to rush the quarterback (they may choose to double-team a receiver instead), however they must audibly count to "4 Mississippi" anyway so everyone on the field knows what's going on.
The "4 Mississippi" count can be quick but it must be audible across the whole field.
Defenses are allowed one blitz per four downs. A blitz allows any number of defensive players to charge in without first counting to "4 Mississippi", as long as someone audibly shouts "BLITZ!". The player blitzing does not have to shout, but at least one defensive player on the field must shout it. Blitzing without yelling the word "BLITZ!" is a penalty.
Fake or phantom blitzes are legal. Defenses can yell "Blintz!" or "Bits!" or "Shlitz" without actually blitzing (beer references are encouraged) in order to scare the offense into doing something stupid. Additionally, the defense can yell "Blitz" and not rush in, although they then lose their blitz for those four downs. Essentially uttering the word "blitz" constitutes the use of the blitz play. Defenses can also have a delayed blitz where they begin counting the Mississippi Count but then blitz.
Notes:
Defensive players who've been drawn offside by a fake hand-off or (if blitzing) by a phony snap count, or by accident, can retreat back across the line of scrimmage and reset while the play continues without penalty. If the defense does not reset, an offsides penalty will be called.
Defenses are allowed one "chuck" of the receiver, using open hands to the body, to hold up the receiver. This can be only be done within one pace of the line of scrimmage (not 5 yards of the line, as in other forms of football).
Touchbacks are spotted 15 paces from the goal line (at the first cone).
On 2-point conversions, the defense can run back a fumble or an interception for 2 points.
No blitzes are allowed on 2-point conversions or X-Point Plays.