Those vocal zapper things aren't very good ... they're mainly for 'fun at parties' kinda stuff.
They work by putting certain frequecies out of phase so that anything dead center in the mix loses volume & cancells itself out. Trouble is, they only work minimally on certain frequencies and the vocal has to be dead center with no reverb. Any other instruments with the same frequecies in the center will also get lowered in volume.
By singing your vocals inline with the same recording, it's hoped that you'll drown out whatever is remaining in the mix.

We used to do a cheap version of this with Lloyds record players years ago when we were learning tunes. You could cross the wires behind the tonearm cartride and it would put the whole thing out of phase. Because the speakers were so small and close together and the sound was so crappy, it sounded like the vocals were being dropped out slightly and you could hear the guitar better.