I have much more important things to do than to read about penis physiology, yet that is what I have been doing, and I have learned a lot, and I think answered some questions that have been lingering here. To start:
The question about whether or not blood flows during peak erection is not so simple. The answer is actually yes, and no!
As pudendum has pointed out, the ischiocavernosis muscle (ICM) contracts, which raises pressure in the CC’s above normal arterial pressure. This pressure can in fact cause complete stoppage of venous outflow.
However, studies have shown that ICM contraction is intermittant:
The belt-form cavernosus muscles’ insertion into the CC presumably constricts on contraction the dorsal penile vessels, with a resulting extra blood entrapment in the cavernous tissue. It was shown in this study that the increase of the cavernosus muscles’ EMG was intermittent, which apparently denotes intermittent cavernosus muscle contraction
The intermittant contractions allow maintenance of pressure, as well as some inflow and outflow of blood to prevent the oxygen deprivation that we have been discussing!
The intermittent cavernosus muscle contraction during rigid erection seems to be advantageous. On cavernosus muscle contraction, the ICP increases to above the systolic pressure and the penile corpora are apparently transmitted into a high-tension closed cavity. Retention of this high-tension closed cavity for long periods during penile thrusting at coitus may lead to cavernosus tissue ischemia. Therefore, the intermissions in cavernosus muscle contractions allow for alterations in the periods of muscle contraction and relaxation during which the cavernous tissue could be well oxygenated, particularly because the periods of cavernosus muscle contraction are relatively short. These factors provide a natural mechanism that keeps the cavernous tissue well oxygenated and prevents its destruction by ischemia
These quotes were taken from the discussion section of this excellent paper: