Without getting to technical:

When you downshift- the synchro is trying to match speeds of the output side of the transmission and the input side of the transmission. The blocker rings (part of the synchro) are very tiny. They are in charge of providing the friction to accomplish the speed matching. Without matched input/output speeds- you can not get the trans into gear.

Thus we have heel-and-toe driving. You blip the throttle while braking (while clutch is depressed) to match the engine speed (trans input speed) with the output speed... This allows the shift to occur and voila! We have downshifted into 1st gear. Its a bit of a performance thing ya know?