Most machine vices will have a tommy bar to aid tightening and releasing the jaws.  There is a very annoying problem with the sliding bar though, it tends to drop down and jam on another piece of the machinery.

So my solution is to slide on a small ‘o’ ring onto each end.

Now you can slide the ‘o’ rings to the best position. The tommy bar will be held securely but is easily adjusted

And now you can spin the tommy bar with ease.