The 'sudden changes' in blood flow need to be paired with a relaxation of the arterioles of the Corpus Cavernosa. This can happen only 4 ways. An erection is a result of blood filling the arterioles of Corpous Carernosa..
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1). Drugs either directly (Muse/Caverject/Befar), or indirectly (Viagra/Cialis/Levitra).
2). Psychological activation from the brain (sexual arousal)
3). Direct Stimulation of the Penile Nerves and/or other perianal nerves.
4). Restriction of the venuous blood return from the Corpous Carvernosa/expand the arterial inflow (ring or vacuum device).
In short, the sudden changes in blood flow need to be mediated by something else to produce an erection. If you increase the blood flow without either restricting the outflow, or relaxing the arterioles of the Corpous Cavernosa, nothing happens.
The Prostaglandins in Muse/Caverject/Befar directly relax the arterioles of the Corpus Cavernosa. Viagra/Cialis/Levitra block an enzyme that degrades a chemical messenger that relaxes the arterioles (produced in response to arousal of some sort).
The Vacuum and ring devices force the filling of the Corpous Cavernosa by increasing the blood flow in, and restricting the outflow of blood from the Penis.