During H-H Homonuclear Decoupling, a proton resonance is irradiated during acquisition. The irradiated coupling is then collapsed in its partner's mulitplet. This allows one to identify the coupling network, i.e., determine what protons are coupled. Since the multiplet is simplified, it can also allow one to measure the J value for the remaining coupling.
The proton spectrum of sucrose at 500 MHz is shown in the lower trace. In the upper spectrum, the 1 proton has been irradiated, collapsing the 2 proton (which is a doublet of doublets from J interactions with the 1 and 3 protons) into a doublet.