stiff leg deads?

I was doing stiff legged deadlifts today and I have a few questions. I have done these before and I know they are great to build hamstring mass, but I'm not sure if I'm doing them exactly right. I did them off a box that was about 6 inches high because I am pretty flexible. Are the knees supposed to remain locked through the entire exercise? And I saw myself not able to let the bar land on the ground in front of my feet...but this is probably because my legs were locked straight. Any suggestions??

When i do em i keep my legs just barely bent. It hurts my knees if i keep them totally locked out. But doing them on a box is good because you can stretch deeper.

just slight bent
like u can barely see it but u can feel it urself
when u get to heavier weight u do that
but when its lil light lock ur knees no damage done i believe