The diffuser doesnt actually create negative pressure though, the high speed air passing under the car creates that (high velocity flow = low pressure, low velocity flow = high pressure in accordance with bernouli's theorem). The low height of the floor relative to the ground basically forces the air to a high velocity creating the low pressure zone. The actual (low) pressure peak is found at the mouth of the diffuser (at the front of the diffuser) and the air below the car is generally below ambient pressure so the whole floor creates downforce. The diffuser serves to assist pressure recovery or the matching of the velocities of the flows where they meet at the back of the car.
Nice explanation.
Now then...who's been following the Jerez testing...
FASTEST TIMES, DAY THREE TESTING, JEREZ
1 Nico Rosberg (Ger) Mercedes 1:17.613*
2 Romain Grosjean (Swi) Lotus 1:18.4193 Sebastian Vettel (Ger) Red Bull-Renault 1:19.297
4 Lewis Hamilton (GB) McLaren-Mercedes 1:19.464
New driver, Hellooooo