The Prawn Suit Grappling Arm is an upgrade module for the Prawn Suit which replaces an arm with a grappling hook which can be fired and will latch onto surfaces to pull the Prawn Suit towards it.
The player can latch onto surfaces as if it were a traditional grappling hook by holding LMB or RMB (depending on which arm the Grappling Arm is equipped on) to fire the hook, have it latch onto a solid object within range, and having the player reel in with the Prawn Suit.
Because the Prawn Suit's jump jets have limited range, the Grappling Arm is a good way to extend the suit's reach. It's especially useful for keeping the Suit in place while drilling for Large Resource Deposits attached to cave walls.
The Grappling Arm has the capability to latch onto and reel in certain Fauna as well. With smaller creatures, this will allow the player to more easily capture them from long distances. Latching onto larger creatures, however, will reel the Prawn Suit towards them, as they have significantly more mass than the Prawn Suit itself. Combined with an arm that can deal damage, this will allow the player to easily scare off larger creatures, and even kill them in some cases. However, this will also allow the fauna being grappled to more easily attack the player, resulting in additional damage dealt to the Prawn Suit.
The grappling hook is a very useful tool for increasing the Prawn Suit's mobility. It can be fired at the terrain (especially at walls/ceilings) in front of the Prawn Suit to greatly increase its horizontal momentum by swinging forwards, even momentarily exceeding Seamoth speeds. The hook must be released before the grappling hook has pulled you all the way to your target, otherwise the suit's momentum will be slowed down as you approach the target. Also note that the Prawn Suit Jump Jet Upgrade is another incredibly useful mobility upgrade which pairs well with the grappling hook technique, allowing you to alternate between jets and swinging forwards to rapidly cross very large distances without landing. An additional grappling hook also allows for faster acceleration, reducing the distance needed to reach full speed.