The Second Man Play is very common in rugby union these days. It involves an attacker or two in a relatively flat position who act as decoys with a pass going behind them. It’s a favourable move because the flat players tend to hold the entire defensive line in place or cause them to pinch in. The player behind either runs into a hole or moves the ball wide against compromised defenders.
The key to stopping it is to recognise when it’s about to happen. Most teams use the decoy player / pod in the midfield and have the linking player either penetrate behind or move the ball wide. It can be used in the wide channels, but it’s not as common.
If a defender can get to the deep receiver, a turnover is highly likely because his/her closest support is usually the dummy runner(s). The first sign that a blitzing defender can push through is recognising either in the passer or in the front runners’ body language that the ball is going to be passed deep. Decoy runners in this case often make the mistake of looking passive and the deep runner may be screaming for the pull back pass. In addition, the decoy runner(s) might be ahead of the passer already, have hands down and/or jogging lightly rather than come from depth at pace and with hands up as someone who’s going to get the ball would.
Defenders pushing from the inside, communicating their support are instrumental in shutting this move down. By calling a ‘Push’ or using another short and specific call word, the defender(s) in front can ignore the decoy(s) and push on through to the linking second man. The scrum half should look out for these kind of scenarios, acting as a linebacker would in American Football, and being on-hand to clean up if a short ball is used or the blitzing defender misses the deep linking player.