Higher H Compatibility will unlock more positions as well as make the the partners become more comfortable with positions that don't match their preferences. Compatibility also reduces the time expended for each position taken; at minimum compatibility 4-5 positions can take up a full period, at maximum compatibility the time cost for each position becomes negligible. Characters with the Easygoing and the Lewd / Perverted traits will increase compatibility faster. Both partners will show a satisfied or dissatisfied face during a position, depending on whether they like it or not.
At "Great" or "Perfect" H compatibility a pair will like even positions that don't match their preferences, although it still won't increase the compatibility faster.
H Compatibility increases slowly with every H and will increase faster by choosing positions which match sexual preferences.