If a football match ends in a draw and you backed the draw at the bookmaker, that bet will win and the lay bet will lose on the exchange.
If you backed either team to win at the bookmakers, your back bet will lose but your lay bet on that team will win on the exchange as they did not win the match.
By placing a back and a lay bet we cover all possible outcomes. When backing a team to win in a football match there can only be 2 outcomes, the team will win, or they won't win. Here's an example.
Event: Manchester United v West Ham
Bet To Place - West Ham
Your back bet on West Ham to win would win at the bookmakers if West Ham won the match.
Your lay bet on West Ham is for them NOT to win the match. The lay bet would win if the result was Manchester United or it was a draw.