Hiking, Scrambling, Mountaineering, and Climbing

If you plan to lock off and need to be hands feee, you should use a descender. Technically, a lock off on a standard abseil belay is an unplanned situation. It's good practice to use a prussik to stop you falling if you let go of your belay strand in an accident though and on occasion I'm happy to take that "risk" to go hands free for a task. Failing that, look up the leg wrap technique. I tend to use a four wrap prussik but that might be overkill if you're using more than a pair of half ropes.

But as I said, if you plan to abseil and plan to lock off to do something hands free, use a descender designed for the task. Prussiks and leg wraps can fail.