Yes, I saw the video before I responded. Really couldn't figure out what you were trying to illustrate. There's a small amount of shifting back and forth by the large pulley, but that could just be the worm pushing on an obstruction and then easing back from it as you release the clutch. Note that the belt is moving along with the large pulley.
Unless badly under-tensioned, the belt cannot slip. It is toothed, as are the pulleys, and it does not stretch. Now, we have seen the odd pulley with loose setscrews, but in those cases, it is a pulley slipping on a shaft (not the belt) and the behavior would be different than what you show in your video.
When you engage the clutch, a small 'brake shoe' presses on the side of the ring gear, and that friction engages the outer shell of a given axis and transmits movement from the ring. With the clutch disengaged, the outer shell is free to rotate, independent of the ring gear (which still remains engaged with the worm). So a scenario where the motor drives the worm into an obstruction, and then you see a bit of bounce-back when you release the clutch, wouldn't surprise me.
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