The sign for wasabi is based on the concept of scraping a piece of wasabi root along the surface of a grater.
Your non-dominant hand is held out flat in front of you, palm up (in a modified "B"-handshape -- the thumb is alongside, not crossed over the palm) as if representing a grater.
The dominant hand is in a flattened-O handshape pointing downward over the base of the non-dominant palm as if representing the holding of a piece of wasabi "ready to grate it."Then you move the dominant hand forward twice (as if grating the wasabi into a paste).
Notes: