I remember a high school buddy who taught himself to speed shift (shifting without a clutch) in a then-late-model Triumph TR7 (a thoroughly rotten car, built during an era when TR meant TRouble). After a little while he got the hang of it. In a strange and coincidental turn of events, the transmission needed replacement a few weeks later.

Moral: If someone wants to teach you how to speed shift, use THEIR car.