Page "Sampling rate" Paragraph 5

Conversely, one may sample below the Nyquist rate.

For a baseband signal ( one that has components from 0 to the band limit ), this introduces aliasing, but for a passband signal ( one that does not have low frequency components ), there are no low frequency signals for the aliases of high frequency signals to collide with, and thus one can sample a high frequency ( but narrow bandwidth ) signal at a much lower sample rate than the Nyquist rate.