From NMR Wiki
Varian and Bruker pulse program code available here
H-X coupling measured as doublet splitting in F1 dimension divided by a preset integer scaling factor (~20). Values of J H-X as small as 0.9 Hz could be reliably determined.
First four pulses together with gradients suppress magnetization from 13C-attached protons, while retaining magnetization of protons bonded to magnetically-silent 12C nuclei.
Assigning couplings in this experiment is straightforward as chemical shifts in both dimensions are those of coupled nuclei.