It means anal sex. But why doesn't it mean 'her on top and him on bottom'? See Tosfos Yevamos 34b, and see Jul 26, 2012. All I could say is that you see from Tosfos that there was no such thing as a 'mashgiach girl' in those days, who would have complained if he put her on top, that this is called "chutzpa" in the Torah.
Actually, today's seminary girls and 'breadwinning' girls, who look down at their husbands - it is fitting for them to be on top, and him on bottom. Just joking; do whatever makes you happy. Just don't be stupid to go to other women. Play with your wife and you won't get punished for it.