MechAlive is a function of the Revenge of the Fallen toys that allows pieces in them to move in robot mode when you perform a certain function. The gimmick is akin to Automorph Technology due to it's use of gears to create a more movie-like appearance. Some toys possess gears and bits that move when you twist another part or move another part. Starscream's ROTF toy has a function where moving his head causes internal gears to move in robot mode. Demolishor's toy has a gimmick where moving the elbow causes his upper arm gears to twist and turn.
So what is the difference between this and Automorph? It's the same slaggin' gimmick! Well, the difference is that MechAlive is merely for aesthetic purposes, and Automorph is a gimmick used to Transform the toy. Some Revenge of the Fallen toys have both, such as Sideways.
The storyline reason behind this Gimmick, since many gimmicks need one, is that the technology was developed by Autobot scientists and was stolen by Decepticon spies later. It is a powerful advance in Cybertronian battlefield technology, since it animates armor to let it deflect attacks, reinforces a Transformer's internal gearing, gives better aim, and allows for more powerful attacks.