What are these???

And this    http://www.youtube.com/watch?v=5-zH_MmoV9w&feature=related

On US tanks, the smoke grenade launchers could be fired in two salvos. Each salvo would launch half the grenades from each side. Their purpose was to hide the tank from guided missiles and were defensive in nature. If you saw a Hind-D helicopter or the telltale signature of a guided missile (like the Soviet Sagger) being fired, you'd have seconds to hit the launchers and hope the smoke would build quickly enough so the missile or its gunner would lose the ability to track you.

Launching them was as easy as flicking up the guard and flipping a toggle switch. You can see the arming switch in this photo I took on the right hand side. The red item is the guard. There is a similar switch with guard in the center of the photo. This is for the vehicle smoke generator that basically sprays diesel fuel on the hot exhaust and creates a large plume of white smoke. The work in conjunction to keep the vehicle from eating a missile.

They do this        http://www.youtube.com/watch?v=qE6B_j3Qxv0&feature=related

Those are usually smoke grenade launchers. They throw the grenades at different angles to get a maximum coverage as quickly as possible. Depending on the manufacturer some can fire a specific tube or bank of tubes and others just shoot the whole wad.