A flaw in the poker strategy of a great many players is predictability. If you can't learn to master the art of changing gears in poker, you are setting yourself up to lose big no matter how much you win with the same old moves up that point.
Changing gears in poker means making a move that is contrary to our outside the scope of your usual poker image. If you've been playing tight, you suddenly become almost maniacally aggressive. If you've been playing aggressively and you are dealt pocket Kings, you limp in. You do this because throughout the course of any Hold'em game or tournie your opponents are doing to you precisely what you are doing to them: scrutinizing your play to get a read on your poker image and your betting patterns. You want to create a poker image, it's true; but you want others' perception of that image to be distorted.
If you have created a poker image of the over-the-top, edge-of-manic aggressive player, you want other players to start saying, "Yeah, but...what if not this hand?" Changing gears means you make your opponents play with more fear and uncertainty than they otherwise would against you because they keep having to think to themselves, "Okay, that guy usually plays like such and such, I know, but...what if he's doing something different this time?"
On the other hand, you can also use this poker strategy in a somewhat different way: deliberately play in the same gear for hand after hand and then, just when you sense you may have won one too many pots with just a little too much ease, decide to slam it into another gear for the next hand. Everyone by now will think they have you read, and instead you stone-cold bluff them after you have them sweeten the pot that you then steal from them.
Have you ever seen an ace fastball pitcher throw heater after heater to a batter and then, on the fifth or sixth pitch of the at-bat, throw him a change up that floats in there at a snail's pace? The batter swings 15 minutes before the ball is even at the plate and looks ridiculous, and likely just got struck out. This major league professional just got made to look like a little league midget by another professional who understood how to throw his timing off. That's how you want to switch gears in poker.
If you're an aggressive player by nature (or through practice and discipline), enter a game or a tournie as that aggressive player and play the way you do. But, when you sense that the time has come, throw your change up in there--make your move of changing gears.
How can you tell when it's the right time? Well, how does that fastball ace pitcher (or his catcher) decide when it's time to throw that big fat change up?
So, you see, it has to do with the situation in the moment. If you have successfully played aggressive poker up til now, and you see pocket Kings before, why not suddenly limp in and keep calling...even after you flop a set of Kings...until you crush them all on the turn or river and take a big pot? On the other hand, maybe you find yourself facing down a couple of opponents whom you know to be loose players, so you decide to just pull up tight and make them think you have maybe a pair of deuces when you've got a flush. Or, you could take a hand or two off from being aggressive, then go right back to it--making players doubt your predictability. And then there is the possibility that you are not alone in being a master of aggression at this table--so you switch off and play the style you find the other aggressive player has the most trouble dealing with.
The most important thing about changing gears in poker is that you can do it seamlessly at the right time, and you can still play well in the gear that's different from your normal one. Your image of predictability will be shattered and players will have to respect you when they make a move--meaning you have greater longevity and more money.