The movement is in the WRIST not in the knuckle.
If you do the movement in the knuckle it tends to look like a
version of the sign for WHO.
Also It helps if you pucker up a bit while signing this (heh).
Also see: WHO
Also see: LIME
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American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars