I don't use a cock ring all that often as being as thick as I am I tare silicone ones very easily and the metal one I use is a good cock ring but it can't "lock" meaning it doesn't give me a very good stretch it can but I have to wear it for a long time and I already have to in the mood to have a raging hard on aka while watching porn. I should give you more info on the monster that is locking, like I said locking is any time where blood can go into the penis but can't come out but the reason I call it locking is because the pressure in the penis pushes the cock ring down to the base of the penis and locks it there this lets the maximum amount of blood in while letting non out. Now that you know that part of locking you know why I'm pushing down on my cock ring in the pic above it forces more blood into my penis giving it the minor stretch I'm looking for. How ever to lock with a cock ring if it's silicone it MUST be smaller then you while non erect and a metal one should be the same size as your flaccid penis and that's why I can't lock with this cock ring it's bigger then my flaccid penis but lucky for me not bigger then me while I'm hard. Now the question can you pump while locking don't do that you'll see if you try out locking that the base of your penis swells and swells while the rest of your penis you know the shaft and head don't, only once you push down on it and make all the blood enter the penis does your whole penis swell and after that it keeps swelling. Putting on a pump would one make your base swell again moving the cock ring you were locking with up messing up your lock, second it would pinch the lock more or less cutting your dick off like the base or your dick exploding and every thing above the lock falling off so don't lock and pump.
PS I'm so sorry for how grim that got at the end there but locking can be a great way to make your dick bigger in fact a little better then pumping but with more risk and less ease of use.
Here is a pic of my cock ring it's almost scary to think I'm wider then it. Pic is copy right of Juggernaut76