[supernewton]

Anecdotally, with practice, some people can flip a coin to a desired outcome like 65% of the time. And .65^20 is only around 1 in 10,000.

[newaccount74]

You flip the coin, catch it, then present it on the back of your other hand.

With a bit of training you can feel the coin with your thumb when catching it to make sure you present the desired side. If you do it quickly enough people won't see you do it. The trick requires coins that have sides that feel distinctive.