As to the blood flow and the effect of the cock ring portion of a CB-xxx, think of wearing just a cock ring that goes around the penis and scrotal sac behind the testicles. When you get an erection, the ring actually prevents the back flow of blood from the penis, which can give you a harder, bigger, longer lasting erection. That is to say, blood has no problem flowing into an uncaged penis when one uses a cock ring.
Now, think of that same cock ring, but imagine your penis encased and completely surrounded by something that constricts the penis and which, because of its firmness, will prevent your penis from getting too much bigger than the interior diameter-circumference of the cock cage or cock tube. As you get excited, blood will definitely flow into the penis. But, the normal amount of blood that should flow into it if it weren't confined has nowhere else to go, so it builds up behind the cock ring. It does NOT stop flowing into the penis until the penis is as hard as it's going to get. If you can find a longer preview of Dom Karin's "Chastity Restrained", about 15 minutes or so into the video, you can see this phenomenon: you can tell that the guy's cock is erect, because it's sticking straight out from his body. The weight of the cock cage keeps it from sticking up more. You can see that his penis is somewhat hard (I didn't notice any of it bulging out of the holes in the CB-3000) and that behind the cock ring there's a huge buildup of blood. It's kinda cool, actually.
It's a sensation that you just have to get used to. First time it happened to me I was shocked and briefly panicked before I realized what was happening. It was very painful, but very, intensely arousing. If your penis looks like this, you're going to feel the pressure: http://www.milovana.com/webteases/showt ... 912&p=11#t
With the Points of Intrigue (the medium length one was used to get this look), if you get that erect, prepare to look like this when released: http://www.milovana.com/webteases/showt ... 912&p=12#t
A lot of the concerns about no longer getting erect are nonsense, nor will it shrink your penis for all-time. The look here is normal: http://www.milovana.com/webteases/showt ... 235&p=12#t
When locked in chastity, I used to say that unfortunately I'm what's known as a "super-expander": lots of blood flow into the penis so that it's 8" long and 1.75" in diameter. But, my mistress says that I should say "fortunately" even when locked in chastity, because it gives her great pleasure to see what an unauthorized or intentionally made erection does.
As to the erection concerns you stated, quite frankly, until you get used to it, you are going to experience some pain. However, what goes up must come down, though when there is some cock ring constriction it can take longer. If you're truly locked away with no key so that you can't remove the device, which you'll probably want to do during a moment of panic, ice cold water can do the trick. So does getting down in a salaam position and begging your partner's forgiveness for having an unauthorized erection.
Your body won't stop trying to have erections. You're going to get them. After awhile you become accustomed to the pain and sensations. If you're out of chastity and do not want one you can slap your penis (not too hard you can break it) or slap your balls (they can take a much harder slap than you think), dip everything in ice cold water or put an ice pack on it (careful that you don't give yourself a freeze burn).
Regular release is the key. THe longest I've been locked up without release by way of an unrestricted orgasm is 30 days. Getting milked or having a prostate massage during that lockup period is just another way of being put in your place, but also keeps the pipes clean. (Prostate massage can cause an injury so reading up on that and being
very careful and having a partner who will listen to you is important.)
Except for no side view showing how erect one can get in a cock cage of any sort, as can be seen in the Dom Karin "Chastity Restrained" video, this will give you a very good idea of what life can be like: http://www.milovana.com/webteases/showtease.php?id=6912
As always, don't try this at home, kids. All scenes were done by professional stunt men or under their supervision. You are warned not to attempt to re-create or re-enact any stunt or activity displayed in the foregoing tease.