750 K0's and (I think) K1's had these, it's basically an adjustable friction lock, helps keep you from having to fight the throttle spring to keep the throttle in one position. As Johnny says it's to prevent fatigue on long rides. Turn the screw in to increase friction, out to reduce friction. If it's too tight then throttle snap-back will be poor when you let go.
Loosen it all the way, and see if your throttle is still stiff. If so then your cables need attention.
See items 13, 15, and 16 below.
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mystic_1