Two opposing players (forward & defender) are challenging for a ball. 

Their momentum carries them off the field of play, just outside the penalty area, below the goal line. 

The ball is still in play in the penalty area. 

The forward gets up first, heading back for the ball. 

The defender holds him (while both are still off the field of play) so he cannot go for the ball. 

What's your call?

The referee should play "advantage" if one exists and caution the defender at the next stoppage. 

If the referee stops play to administer the caution, play is restarted with a drop ball because the incident took place off the field of play.