Some time ago I came across a version of a sign for WHEN? in NSL.
As this sign has to do with time, it is centered on the right cheek.
The handshape has the three first fingers at right angles to one another, the last two being bent into the palm.
With palm facing back, the middle finger strokes the cheek up and down, really fast. Hand is not moving, nor is the index or the thumb.
Here are two attempts to write this:
I am not at all sure that this is clear