Mizu sumashi means Literally: whirligig (an object that spins or whirls) however it usually is translated as “water strider'” (or “water skeeter”) which than again should be translated as AMENBO in Japanese. To make it worse: It seams that mr. Ohno once has made a minor mistake (Yes even he was not perfect…) and translated it as a water spider (a name you can find in many companies now…). The purist (like at Boeing) would call it a ‘water strider’.

However, a long story short;

Logistic function in a pull system; the person who delivers materials to the line and picks up its completed products.

You would probably expect it to be a low-qualified job… a real water strider is a very skilled person who has a broad knowledge about the process he/she is ‘feeding’.
Since in a pull system there is only very limited material at the line, the water strider immediately stops skeeting when there is a problem somewhere; this person then has the task to help get things going again, a kind of first aid so to say.