There are three actuators possible on a mech's arm: upper & lower arm actuator and hand actuator. Hand actuators don't make any sense in MW:O as there aren't any melee weapons and melee itself isn't possible (yet?). The arm actuators are responsible for the mech's arm movement - the upper for the movement up and down, the lower for pivoting the arms to the left or the right. Upper arm activators are on EVERY mech in the game while both the lower arm and the hand might be disabled on certain variants or chassis. As a part of balancing clan tech vs. IS tech mounting a clan autocannon or cERPPC prohibits mounting a lower arm actuator. If you own a clan mech you can easily test the effect of the lower arm actuators yourself: take an arm omnipot with energy hardpoins only, mount some cERMLas and do two rounds on testing ground, one with the lower arm actuators installed (you might need to tick a box to use them) and one without. Note that you need to disable arm lock to recognise any difference at all. EDIT: Note that you can somewhat 'cheat' on the rules regarding clan ACs / cERPPs. As you're not allowed to operate two seperate reticles for the arms, you need to mount at least one arm with lower arm actuators to automatically get an articulated arm for your cERPPCs or cACs. See my examples below. My favorite Summoner: Dire Wolf A (stock):
Basically there are mech parts that take up your arm slots. Shoulder + Upper: These are standard and basically all mech have them. They allow for that extra up and down of arm weapons. Lower: This allows for weapon swing extra left and right. Hand: This does nothing and is a complete waste of slots. (at least until they allow us to give each other the bird or use melee attacks and hold poweraxe and chainswords and what not)
I'm pretty sure the hand actuators do help with some 'Mechs, maybe not via an actual in-game stat or anything, but because the developers are aware of the hand actuators, and have given several of those 'Mechs more arm swing, in all directions. The HBK is a prime example of this. It's arm mounted(hand mounted, essentially) laser weapons have an extreme degree of swing.
