The strategy is called 'market timing'. The reason we don't recommend it is simple. We can't see into the future. Unless you're really sophisticated, really talented and really good, you'll miss many of the highs and lows. And that will eliminate any advantage you might have gained.
Some people try to use a variation on marketing timing.
That way no matter which way the price goes they'll be right with part of their money. This works much better than being completely invested or uninvested. Finally, M.A. asked about moving into something less volatile. The time to address that is before you choose your investment. You need to decide what you want that investment to accomplish before you put money into a mutual fund, T-Bill, CD or any other investment.