Clamping with Jelq sticks : safetly

I just started this routine, I am going to have to search to learn more before I do more.

I am just curious for those who clamp , it seems very risky to me Iam no doctor, but you are putting pressure on the valves the keep the blood in your penis .?

First of all, to counter act this pressure on the valves, I use jelq sticks to push pressure towards the head. Any body do this ?

How do you achieve being able to keep the outgoing valve ‘blocked’ , and allow the incoming valve to stay open ? Are they located on the left and right on the base respectively ?