It's a kick ass semibluff, it works because if you get called you have outs. In this situation the donkey with the 5-2s is thinking that he has nine diamonds to make a flush(he doesn't know hero has Ad) and three cards to complete his gutshot which gives him almost a 50/50 shot to win (I think it's like 48% or 49% to make his hand, I suck at math). So, he has an escape hatch if called. I use this move quite often. However, you also need to figure in things like stack sizes and whether your out are clean, etc.