Most skills take a certain measure of practice to master...unless you are insanely smart. Need to make a million-to-one shot to stop the Doomsday Device from exploding the world, but have never even fired a gun? Just run off some mental calculations about your gun's firing speed, friction, gravity, and the slightly off-kilter scope, and it's a done deal.
It's like a Xanatos Roulette
with physics...and usually pulled off by the same Magnificent Bastard
. In general, though, it's any situation where calculation takes the place of practice.
Example: In the Disc World
, Lord Vetinari pulls off a juggling act despite never having attempted it before.