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I think the rotator type cap and keys combination do cause valve rotation.  Here’s why I say that.

The cap and key combination work to push the keys away from the end of the valve stem when the valve is opening allowing the valve to move independently from the spring.  (The inside of the cap is deeper than the distance from the valve tip to the keys.)  The rocker is pushing the cap against the keys and upper spring seat, then the valve tip hits the underside of the cap, but it is not being held by the keys to the spring upper seat.

The geometry of a coil spring is such that as it is compressed the ends rotate a little with respect to each other.  So as the valve is opening the valve goes straight with no rotation and the spring upper seat has rotated slightly.  As the rocker arm moves to let the valve close the spring upper seat pushes the keys against the valve groove to pull the valve closed and forces the valve to rotate with the upper seat as it undoes its rotation from compression until the valve is closed.


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