I have bought a bits set and there is this bit:

It is a Y shaped bit, and there is a groove on the inside of the branches. The angle is approximatively 90˚ (a little more).
ACCEPTED]
It's a bit for screwing in eye bolts [1], or hooks.

The list of bits in this set [2], lists it as a "Y-shape hook driver".
[1] http://en.wikipedia.org/wiki/Eye_bolt
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Also can be used for wing nuts.