sampling below nyquist frequency
Nyquist gives you the minimum sampling rate so that you can determine the frequency of a periodic wave.

If you sample a sine wave at 0.75 Nyquist, you get a false sine wave as shown in black dashes.  This is called aliasing and it occurs at any frequency below Nyquist.



At Nyquist, you can determine frequency, but not amplitude, of a sinewave as shown by the black dashes.

Almost all images of Nyquist on the web show sampling at the peak, which is almost impossible.  The following is a more appropriate representation.  



Above Nyquist, it is possible to see frequency and amplitude of a periodic waveform by repeated samplings.  At 1.25 Nyquist: