When Shooting from a Bench, Rest, Lead Sled, or Sandbag You Must Hold the Fore-Grip Tightly with Your Off Hand
Pass-thru fore-grips are a big advantage with this type of shooting since you may be able to get most, if not all, of your hand through it. By putting your hand inside the fore-grip you should be able to put a good amount of downward pressure on the fore-grip to keep the front-end from bouncing when you shoot.
When Shooting Off of a Clamp-On/Clamp-In Style Rest, Hold the Front of the Crossbow Securely
Just bringing the crossbow back into and tight to the shoulder isn’t going to stop the front-end from bouncing. If you can’t grip the fore-grip, make sure you hold the bipod, tripod, etc. securely.
Shooting offhand there’s not as much “bounce” since you are already holding the crossbow at the fore-grip and pulling it into your shoulder.