Cold showers create vasoconstriction, which consticts blood vessels, which DECREASES blood flow... and of course the vasodiolation, which expands the blood vessels, INCREASE blood flow...
During exercise there is vasodiolation... so, the idea of a 'cold shower' is to create, vasoconstriction, to help cool down the muscle. The contrast back and forth is to act like a pump which helps send blood to the muscles to help in repair. This techinque is more so used in rehab...
As far as working out and if they help... personally from what I've read/tried, it doesn't really make a difference.
(sorry about spelling errors, it's 3am and I can't think straight )