I was teaching a workshop once upon a time, and felt the pianist was
going a little fast for the conditions. I gave her the hand signal I had
used for many years (learned in a radio-studio context) for "slow down,"
i.e. a pulling apart of the hands as if one were stretching taffy. She
gave me the darndest look, and if anything speeded up. The class went on,
I was at the far end of the hall, and I tried again to slow her down, and
the music definitely got faster. I switched to words rather than hands,
and it all sorted out fine.
It turned out she had never seen the like of such a hand signal before,
and had interpreted it as something quite fascinating but I'm not going
to tell you what it was!
Anyway, I *highly* recommend agreeing with the musicians just *how* you
will tell them to go slower or faster!