Defensive Offsides - How does this rule work?

[landrun]

I've seen this penalty called different ways and am not sure how it works.

Sometimes the offense gets 'a free play' but other times it is whistled dead and the play is stopped and yards walked off. (just happened in the missouri/SC game)

How does this work??

[S.PadreIsl.Sooner]

If the defender has a clear path to a ball carrier or QB is when it is generally killed. Or contact made before the snap.

[BoulderSooner79]

Unabated to the QB whistles the play dead. So if you jump offsides, don't bring your bait along or they get a free play.

[ashley]

I coached for many years and haven't figured it out yet.

[jkjsooner]

Padre nailed it.