Check out the full guide here Atavism IK Handling
The IK Handling adds a series of options in the Display item for the particular piece of gear. This allows you to set the value for the IK Handler right into the Equipment Display Item and have it apply to the character in game. When the character loads the gear, it will load this and enable or disable items based on the toggled settings.
You can create Weapon IK handlers, typically the left hand is more than sufficient to use for this purpose. This be because most of the time, the animation already holds the position of the right hand correctly, if it’s a right handed object. Reverse this for left handed objects. So for this type of item, we would just add a Left handed weapon IK item.
Don’t forget to add IK pass to your layers where you need IK Handling.