The details are still pending, but generally it’s pretty simple. If you’re waiting for a seat, you can place a bet (probably $5 min) on another player. You follow their decisions, so if they decide to split 4s against a Ten, you’re on the hook for two bets.
You can also bet on another player if you’re seated at the main game, doubling your action. It’s a pretty cool feature, and makes waiting for a seat less annoying.
Should launch soon on Golden Nugget and Betfair casinos.
Bet Behind sounds like a fun option to try. I have friends overseas who are able to play Bet Behind in a couple of foreign online casinos. (However, I live in New Jersey, so I can’t play there.) What they like about Bet Behind is that when they are on a losing streak and they see another player winning hand after hand, they can put their own chips behind those of the winning player and try to get a piece of the hot streak, too. Of course, it doesn’t always work, but I imagine many players think it’s a good way to try to change their own luck around and that’s why the bet is so popular. Also, as you say, it helps kill the time if the table is full and you’re waiting for a seat.