Imagine tapping a 4" square piece of 1/4" thick steel...
Drill the appropriate sized hole in the steel. Place the steel on the base plate of your tapper. Insert the appropriate tap into the upper gizmo on the tapper. Lower the tap to the hole in the steel plate. Move or insert that screw in the base plate so that it restricts the rotational movement of the workpiece. You don't want the workpiece hard clamped, you want it to "find the bit" and be able to move so the tap will self center. But you do want the workpiece to keep from rotating so that the tap can do it's thing.
When you want to back the tap a half turn to clear the chips, I guess you'd have to manually hold the workpiece in this style tapper. Some have a sort of floating vise that restricts rotation in either CW or CCW direction.
I have the Chinese style they sell at Enco and other places. I think it's one of the most used tools in my shop. Much easier to setup and use than the drill press method.
Limited, of course, to the size of workpiece you can fit in there.
Lump