So there are certain ground-pounding moves which in CRITICAL MASS turn into some heavy massive attacks, but i cant figure out what they mean with CRITICAL MASS..

The health bar in the top left hand corner of the screen is red, however, when you gain enough health - by consuming people - a blue bar will be added to the end of it. With this added blue bar visible you can perform your critical mass abilities, but be quick about it since it still counts as health, so if you're damaged the blue bar will go down as well.

I also think it works when you're low on health and the screen turns grey, but I think you need to buy the upgrade for it first.

when you get to the blue bar of health you can press and hold b+y or x+y to do a special attack. This is very usful on the last boss.

An interesting note about it, the more blue critical mass you have the more damage that move does.  theres a point midway though the game where two of these can take out a hive ur standing on, (later in the story all the buildings seem to get fortified though)