It's a torque gauge. You put a bar through that hole to hold it steady, socket on the square drive and use a breaker bar in the female square drive on the top. Instead of being click-type it's got a gauge and you use a bar to apply force and a reaction bar to hold it steady.
As mentioned its a Torqometer. Here is Snap-on's version.
http://buy1.snapon.com/catalog/item...group_id=966&supersede=&store=canada&tool=all