i've seen, as in this video http://www.youtube.com/watch?v=dazNITsgEog that some alien weapons have a sort of secondary fire, for example the pinch opens itself and has a strong short range fire.anyone knows how to use it?

Toggle the firemode

I think the default is by pressing 2

press "2" but not all alien wep's have it My favorite is X-Pac Mortar with his extreme charge as secondary fire mode xD