"Manual" clamping
This isn’t really clamping at all (yes, sorry for the false advertising ;) ) but I’m thinking it could have a similar effect.
Get a hardon while lying in bed. Then lie on your side facing slightly diagonally downwards. The pressure of your erection will increase due to gravity causing extra blood to rush into it - not as much pressure increase as a clamped dick, but extra nonetheless. I did this yesterday and it felt fairly intense after a minute or so, causing me to stop because I am supposed to be on a two week PE break ending some time next Monday.
Any thoughts on this? Could it be classed as a PE exercise, or even a mild form of clamping?