well I just consistent FS 360s and I'm kinda concerned on how I do them. I do the whole setup carve and go off the lip on my heel edge. Thats fine but when I actually do the spin I throw my weight and right as I get to 270 I stop. Then I do a shifty and land it fine on my toe side edge. This is the first part that concerns me the second is that I really want to learn FS 5s and I'm having trouble rotating past 270
The way I understand it, your body as a whole is not really "stopping at 270" as that would be impossible. You can "slow" it if you like stretch out your arms...similar to how a diver goes from a ball to straight (i.e. increase moment of inertia), but this is prolly not what's happening here.
Perhaps what you are doing is that at 270, you are twisting your head and shoulders toward the direction of travel quickly...in order to look at the landing perhaps. What this can do is make your board twist in the OPPOSITE direction. But since you're net rotating, it's "stalled" relative to the earth. Does that make sense? Newton's 3rd Law: Every action has an equal an opposite reaction.
So (if regular) imagine you are just doing a straight air and your upper body all of the sudden spins CCW. The "equal and opposite reaction" is that your lower body will spin CW simultaneously. You can try this sitting on an office chair. Now make this a spinning air at the right speed and the same scenario...a "faster upper body rotation" causes a "slower/stalled lower body rotation".
The grab would definately help prevent this upper body jerk if this is the case. Either that or just try to keep your shoulders and arms still and just look uphill while landing. It's called landing blind. I mean, not blind as you close your eyes, just look toward the rear of travel.