From NMR Wiki
This experiment[1] works for molecules whose T1 relaxation time is shorter than T1 of water.
In this experiment tn is selected so that the Z magnetization of water is zero at the moment the 90o pulse is applied.
References
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Patt, SL and Sykes, BD. Water eliminated Fourier transform NMR spectroscopy. The Journal of Chemical Physics 56:3182, 1972. BibTeX [weft]