What does “bet behind” mean?

When a blackjack table is full, some live blackjack tables will offer you a chance to play together with one of the players occupying the seat. You get to be in the action, but he makes all the decisions.

You do however have an option to pre-select whether you want to split and double together with that player.

Given how many tables are offered by Evolution Gaming,we don’t see why betting behind should really be something that you need to do often.