When a team is awarded a penalty near the in-goal, say at 5 yards, and they play a quick tap. How you defend against that? I have seen yellow cards shown for tackling the player (for example, against Argentina when playing the All Blacks at the World Cup). In that situation, what are you supposed to do? How do you avoid the try?
Since the entire point of the quick-tap penalty at short range is to make it nearly impossible to prevent the attacker crossing the line, the defence has two legal options.
Pull back to the line to ensure onside, then group-tackle: this prevents further penalty and provides the best opportunity to either turn over possession for clearance, or slow down the quick play so the defence can regroup.
Force a held-up: a riskier tactic, but if the defenders' scrum is strong or can even drive over, potentially worthwhile, especially as the defending players only need to prevent the grounding alone, and not the forward movement as well.
As the offside line is only the goal line in this case, any player on or behind the goal line may tackle the player who has taken the penalty and stop a score.