If you are unable to dig under your opponent’s defensive arm initially, you will need to release your grip on the Seat Belt or Harness and give a defensive opening to progress. 

The one thing we want to try and avoid as the attacker is our opponent getting a cross grip on the wrist of our attacking arm, this is one of the reasons we always cover the hand/wrist of the attacking arm with the underhook arm when using a Seat Belt. The inevitable problem is, we cannot attack our opponent’s neck without releasing the grip we have covering the end of the attacking arm, thus presenting an opportunity for our opponent to control the end of the lever. This is where we need to understand efficient grip breaking mechanics  to free the attacking arm again.