Lock?!?!?!! whyever for?
Anyway here is some technique that may help. If you shit on the backstroke, it gooshes out all over and can get on your dick, even IN your dick. But on the pro side, it makes your tit fucking a lot smoother, it acts as foul smelling lube. But if you shit on the forward stroke, it doesnt fuck up your stroke, but its like a big warm pile to scoot back in to at the back end of each stroke. Kind of nice feeling. So the choice is up to you really but i recommend the forward stroke