Jitter is just a sequence of buttons that can make a burst fire weapon and some shotguns automatic! You just get the macro, (THERE IS A DIFFERENCE BETWEEN TAC JITTER AND RT JITTER!) assign it to either a tac switch, or the right trigger, if its on a trigger, I recommend that you make a toggle with one of the tacs. Anyways, then you just hit the trigger/tac and the 3burst gun or the shotgun will fire faster than it is supposed to do! Its amazing at close range. Especially if you use the Spas-12. Its fully auto!
Anyways, in case you're wondering about XYY and YY.
YY slow/medium is better for 3 burst guns like the M16 and the G11.
XYY Medium/Fast is better for shotguns.
SLOW/MEDIUM/FAST are just slightly slower/faster than the other, and they are used for different guns, its personal preference, try them all out if you'd like.