during a set scrum is one of the locks allowed to slip his binding of his second row partner and drop his shoulder allowing him to reach into the scrum as a wyn-jones did last week
The Rugby Ref didn't see the incident you were referring to, but locks must maintain a continuous bind with each other and with the front row for the length of the scrum. Handling the ball in the scrum (if that's what happened) is also illegal.
20.3(f) Binding by all other players. All players in a scrum, other than front-row players, must bind on a lock’s body with at least one arm prior to the scrum engagement. The locks must bind with the props in front of them. No other player other than a prop may hold an opponent. Sanction: Penalty kick
20.9(b) All players: Handling in the scrum. Players must not handle the ball in the scrum or pick it up with their legs. Sanction: Penalty kickThanks
The Rugby Ref