It updates the word list underneath the search bar with every letter typed...at full speed typing. Since typing speed pretty much defines the maximum data input speed, I think that qualifies as 'incredibly fast'.
Still cool, but I'm not sure it's much faster (Assuming you don't care about the suggest functionality).
Otherwise, the differences are more about presentation than an actual difference in the technology. I guess I was really just impressed by the user interface of Definr.