It is an athletics trained action to disarm someone. Under its rules it says you can knock an item out of someone's hands (into their space) with a critical success, a success loosen's their grip on the item, a critical failure makes you flat-footed.
The disarm weapon description says you can use it with the athletics skill even if you don't have a free hand. The question here comes in that it says on a critical success you still need a free hand if you want to take the item.
Is this missing a step in the book, or is it a change that got made one place and not the other? The normal disarm doesn't let you suddenly hold onto the item, but instead of it drop in the enemies square.