.....belongs to the opponents! 

Here's a great example of this bridge phrase in action, from an online league match. The players made several excellent bids at various stages, but then fell at the final hurdle, which was where it so often is, at the 5-level.

East opens a normal 1♠  with their attractive hand, and South makes a very good 4♥ bid, making life as difficult as possible for the opponents. 

West then judged well by bidding 4♠, and North continued the good work by recognising that:

1) 4♠ was very likely to make

2) 5♥ doubled was likely to be cheaper than -620

3) the opponents might bid on to a failing contract

East then fell from grace by bidding 5♠ . The hand is strong and balanced, and much better suited to defending. The Q♥ is likely a wasted 2 points. OK, the penalty might not fully compensate us for our +620, but it will be a decent plus score. Unfortunately, we have now converted it to a minus score as 5♠ is doomed to go down.

Fortunately, South came to E/W's rescue with a 6♥ bid, which is difficult to justify, and was promptly doubled for -800 and lots of IMPs. Leaving the opponents in 5♠  would have meant +10 IMPS instead of -7 IMPS!