This drill is a good development for building in holding your body shape under pressure - do not use this as workout for the two players. It is designed for the single scrummager to work on technique whilst under pressure.
This exercise allows you to develop good body shape in all forwards - they get multiple opportunities to practice foot positions and correct body shape and movement.
Scrums are a great platform to attack from in seven's, notice the fly half (2nd reciever) is directly behind the scrum and can choose which way to attack. Key factors : Work as a unit - Strong core - Tight binding - Leg drive
A great start for learning about correct and safe body shape in beginners. Also used with advanced senior players for effective scrum preparation and warm up.