The Joist Jaw is a solid steel crossmember that clamps to a joist and creates a metal “lip” to hold it in place while you work. Think of it as the “second man” you wish you could afford to help you with the framing job.
Besides just holding it in place, the Jaw also levels the crossmember with the rimjoist or ledger. Three small studs on the inside of the channel bite into the wood while the cup on the end of the vise-like screw clamps down on the beam with a few twists of a handle.
One sad note: they’re a bit expensive. At $50 a pair, you might be better off just calling a friend and promising him beer after the project. Of course, the Jaw is probably more reliable — especially if you have a lot of framing work on your schedule.