The only easy way to show it to someone is to get them in a car that has a trans with complete toast for syncros. On an up shift getting the gear is a matter of timing and/or listening to engine speed. The speed of most people's clutch foot and how fast the engine's RPMs drop combine to make double clutching a pretty useless ritual. If you have a massively heavy flywheel then that makes for a different story but it still isn't required.
When downshifting with the same 'toasty' syncro combination you will never get it to downshift without double clutching and a very accurate throttle blip.
The real problem is double clutching will never work right if you have to think about it. Get a crappy car (
preferably with a box full of marbles for a tranny) and practice until your brain is no longer involved.
(
Here is where being so cheap has paid off!!!
)