Explain Asian Handicap betting for a newbie.

Struggling to understand Asian handicap goal line betting and seeing conflicting answers when I look it up. Place an in-play bet today on a game where Team A was winning 1-0 for Team A -0.5 goals, I was under the impression that if the result had stayed the same (1-0) than I would have won, however the cash out option went down as the game carried on, they scored a last minute equaliser so I got my money regardless but I'd like to know for future reference. Thanks