Exchanging nearly all man in draughts (checkers)
He has 14 pieces and his opponent 16. So his strength compared to his opponent is 14/16 = 7/8. 

The exchange is the trade. Trade 13 pieces for 13 other pieces. So he has 14-13 = 1 and his opponent 16-13 = 3. So his opponent has three times as much pieces as he. The exchange has nothing to do with the promotion to king.

When you are behind, it is often not advantageous to trade. Unless you get a better position.