What does -M1 mean on the bar?
Eval bar
I've been wondering, is it possible to enable it while playing yourself?
well, you could. just put the moves into an analysis board and open it. but that is cheating.
also if you're talking only eval bar and not best moves (lines), that would make the player think more if it's mate in one.
how do livestreamers ave eval bar constantly tho
They don't on their screens or shouldn't.