It's what I would call a hinge boring router bit.
It can make a flat bottom hole or a through hole about 25 mm deep. It's meant to spin a lot faster than a typical forstner bit, which are meant to be used with drills.
Yeah I didnt kno what they where called! I just said forstner bit as I didnt kno the name of the router version of them, just look the similar -ish but obviously with a shank which would fit a router and can handle higher speeds.
You see I would of just gone to my local dealer and looked in his large display or gone internet searching to find a ''forstner'' hinge boring router bit. I should of looked on the internet for the proper name before I gave my advise to be honest! I was being lazy!