It's a little plastic airfoil, that replaces the fletching - made by company called Starrflight. It fits right at the nock end of the arrow and is actually retained by the nock. They are known for great long range accuracy with broadheads. One down side, though, is that they come off (along with the nock) when the arrow passes through something. Most folks use a very bright cresting so they can find their arrow. Something I thought I would try that didn't work out . . . . .