Well,  it's not very expensive - for this sort of tool.  Nonetheless, it is very low quality, not a precision instrument.  Pieces do not fit well, if at all.  This tool has three sockets to insert metal 'pegs' into, that can be adjusted for width of the watch, for removal of screwed-on watch backs (only - not for snap-on/off watch backs at all).  This is the crux of the problem:  the pegs do not fit into the receptacles, uniformly, if at all - meaning that when they can be forced to fit, they are not at the same level (which they need to be to fit into the indents on the back of the watch that you hope to remove.  The tool is just poorly made, its a toss of the coin as to whether it will serve your purpose, or not.  I returned mine, with no problem - thank you Amazon for that.

Also, nothing to do with this tool, you should have some sort of holder for the watch that can be a piece of wood clamped firmly onto a workbench, with two nails in it positioned to firmly hold the watch in place.  The watch, being placed crystal down, should also have some protection from scratching or marring the crystal - like a soft cotton fabric placed between the crystal and the wood.  Some watch backs are very tightly screwed on, and require considerable and careful force to loosen them.