Modern spectrometers use digital filters to eliminate signals outside the desired sweep width in the detected dimension, but signals in indirect dimensions wrap back in. With judicious choice of sweep widths and parameters this can be taken advantage of. The unaliased spectrum of strychnine shown below was collected with a 13C sweep width of 160 ppm centered at 75 ppm. The aliased spectrum was collected with the 13C sweep width reduced to 80 ppm and centered at 50 ppm. The number of slices collected in the aliased spectrum was reduced to half that of the unaliased spectrum, resulting in a spectrum collected in half the time, yet with the same resolution and signal to noise.
When setting up an experiment with intentionally folded dimensions it is often useful to test a few different values for the sweep width and offset until you get the peaks just where you want.
Aliasing occurs in spectra recorded using the States quadrature method. The TPPI quadrature scheme "folds" resonances rather than aliases them. Where aliasing wraps the resonances around to appear on the opposite side of the spectrum, folding places the resonances close to the edge that they were excluded from. See Figure 7.4 in Protein NMR Spectroscopy by Cavanagh et al for a nice schematic of the difference. All of the standard parameters in use at the Skaggs NMR Facility use quadrature methods that will alias resonances rather than fold them.