But the idea isn't just one market, it's supposed to be rebalanced according to whichever asset class is cheap, so you catch the upswing of that. As to how that works, I'm curious...
I guess basically you use a rule of thumb like "your age in % in bonds, the rest in stocks" (only a bit more sophisticated) and re-balance in periodic intervals.
Still you are subject to the ups and downs of the market(s).