Artist's comment:
Kamo-no Irikubi (Duck Neck)
The woman lies on her side with her back pressed against his front. The man lifts up one of her legs and penetrates her from behind. The squeeze the man gets reminds one of how slender a duck neck can be.

"The squeeze the man gets reminds one of how slender a duck neck can be." What, like, getting a deepthroat blowjob from a duck? Must say I've never tried THAT before.