If the penis points downward, with an 'n' shape, then it hinges along the top.
If a penis points upwards, with a 'u' shape, it hinges along the bottom.
'n' shaped dicks have a shorter measurement along the bottom and a longer one along the top. The shorter bottom length goes in to the hilt, and then you can also lodge the delta between bottom and top in.
Same with 'u' shaped dicks, except that its reversed. You go on to the hilt on the top side, and then can also insert the bottom part until it is not geometrically possible for more.
I can get in up to the pubic bone and then have an extra 2" which can be used to ram the point home, which is very useful for hitting such spots as the prostate when he is on his back and the g-spot when she is on her back.