Google Instant is one of those simple but surprising technologies that Google brings to the Search Engine market from time to time. Essentially, the technology allows the engine to show and change the results dynamically while you are writing your query, leading to important efficiency improvements in query formulation (as justified by them...).
Incidentally, I have came across the same functionality at the DBLP Computer Science Bibliography, where it is called "ZOOMED" and not explained (as far as I can find... in a while and without asking XD).
For instance, a search for my name after the first word outputs the following:
You can see that there are 15,136 results in the "zoomed" area above, that go into 1,467 after entering my second given name:
We get 76 and finally 19 when entering the third and fourth words:
I wonder how long has this technology present in the DBLP, I guess before Google (but I may be wrong!)... Anyway, congrats!!!!