carpediemracing 

Find a rider that uses the small-small set up. Find another rider with the same large chainring and large cog that uses the big-big method.

Put it in the big-big. Lift the rear wheel off the ground. Turn the pedals. There's a lot of friction in the big-big when using the shorter chain setup.

It also works if you do the small-small method on your own bike then the big-big, but then you'll want to go to the small-small and you'll have to lengthen the chain.

psimet also makes a good point. It's one of those safety habit things. You always wear a helmet or seatbelt when you ride or drive. You don't plan on crashing either vehicle when you start off (unless you're committing suicide, murder, or you are a stunt person), but you still do it. Using the small-small method is very safe and eliminates accidental wheel popping out (or worse) when you inadvertently use a wheel with a larger large cog (heck, the second cog may be bigger than your current big cog).

The extra chain wrap you get when you use the small-small method also gives you quicker and/or more precise shifting in the rear. The chain has less distance to span between the pulley and the rear cog. Before all derailleurs had similar geometry this was the way to get less-functional derailleurs to work well (or well enough).

The only disadvantage to the small-small method is that the chain bounces more in the small ring and smaller cogs due to lower chain tension (a result of longer chain and a relatively light spring in the rear derailleur). If it's a big issue you can just put it in the big ring - that's an acknowledged way of dealing with rough terrain, using the big chainring to increase chain tension.