He means that your dominant hand keeps the time, while the other hits the offbeats. I thought that was rather obvious.
I'm trying to suggest that the dominant hand wants to be in charge and driving the rhythm, and this explains not only why string instruments settled on a right handed-pluck/strum/bow design, but also why some (most?) left-handers prefer to pick/strum/bow with their left hand.
It would also explain why some (most?) left-handed drummers (including conga drummers, apparently) prefer a reversed set up.
I'm categorically NOT saying that the other hand does nothing. That's just a strawman.