Here's the best explanation I've seen:
My understanding is there are two frequencies involved. One is the frequency of the signal, and the other is the data rate which is much low.
When you push the button on the remote, it starts sending a bit, either a 1 or a 0. These might be different amplitude, frequency or phase, but in any case, the bit is held for a large number of periods of the signal. During that time, your head starts to oscillate as an antenna at about the same signal frequency. The car is far away, so it sees the remote and your head as reinforcing the same signal.
Eventually the bit changes, and after a few periods your head is now repeating the new signal.
Net effect, your head improves the remotes range.