Once the ball touches the ground in the tunnel, any front-row player may use either foot to try to win possession of the ball. One player from the team who put the ball in must strike for the ball.
Why? To promote scrum stability, a fair contest for possession while also giving the advantage to the team throwing in (non-offending team).
It is clear that opponents do not have to do the same and can employ an 8 man scrum as soon as any of the opposing strikes for the ball.
Stability? World rugby declined to answer,claiming too busy, to reply!