How you should hold a multi-tool rasp depends on the type of multi-tool rasp you are using and which tool on the rasp you want to use.
Multi-tool rasp with bottle opener and panel lifter
When using the rasp on these multi-tools you should grip the tool on either side of the protective guides of the body between your thumb and fingers.
The wedge of the panel lifter is pushed under the plasterboard sheet using your foot. Then, by placing your foot on the bottle opener end of the tool and pressing down, the plasterboard panel is levered up.
Multi-tool rasp with a plasterboard saw
With the plasterboard (jab) saw folded out and locked into place, you should hold the multi-tool with the handle in the palm of your hand just the same as if it were a kitchen knife.
When using the rasp on this type of multi-tool, the handle is held in the same way as when using the saw, so you still hold the handle as you would that of a knife.
