G'day Elis,
The flash back production can be tricky, but with practise you'll be able to nail it. On the CoinOne DVD, Homer explains that originally, he performed the move with just the downwards motion; but found (and instructs you to do so) a slight upwards motion and then a swift downwards motion works best.
Also, make sure you're not clipping the coin too tightly. As there is another coin on top of it, the coin held in DB clip doesn't need to be held so that if there were no coin on top it would be completely hidden. You can have a fair amount of the coin right between your fingers, and therefore won't need to grip as tightly.
It may also help to open your middle and ring fingers slightly, on the downwards motion.
Try these tips out; with practise, you'll have no trouble performing this production.
Nick