I often play with friends, so we create a private room to play/train, let's say I was teaching my friend to do a move, I tell him the instructions but he forgot to move something that he should, then instead of having to suicide or kill him, I simply just /reset the game, nothing wrong right? But sometimes when I /reset, it just does the starter move both of us used in the last match, we don't need to press nothing, neither the space-bar...

Usually it happens. Try to not type /reset often.

I think this happens sometimes when both press the spacebar fast after resetting/ playing again

It's a glitch. It happens sometimes.