My simple question is ... how does one practice that ... and how does one have enough confidence in it to actually do the move?

Watching the Rumble live for the first time, I became convinced that the Rumble isn't booked to the level of detail I would have expected. There were too many moments where it made little sense for a wrestler to be knocked out when he was, as opposed to a few moments later. Diesel is a particular example, unless they were trying to get the stare down moment in particular.

Nash coming out as Diesel especially doesn't make sense considering he was Nash just a few years ago as part of the nWo. But, then again, the WWE seems to assume we don't remember what happened more than a few months ago.