Problem: At high rpm the large impeller blades create too much pressure and the impeller (or the drive gear) will spin and can be driven off the shaft.
(Just for reference, it’s common practice in the Cummins world to cut down the impeller blades, decreasing the pump's efficiency by not letting it push as much water. I’m not sure if that’s common practice in the Duramax camp though)
Although the general consensus is that an electric water pump is the best solution in a performance application, keying the water pump is a cheap fix to a common problem found in Duramax engines pushing 4,000 rpm or more (and it’s highly recommended at these engine speeds).
Fleece simply drills a ½-inch deep hole where the shaft meets the cast impeller assembly (half-mooning the impeller and the shaft), and taps a roll pin into the hole (the process is repeated on the other end of the shaft). The result is a solid solution to performance-related water pump failure.
*More on water pump failure from Jason Thompson here