In general, niacin increases blood flow, causing the flush, to the surface of the skin especially the face scalp, chest. After a person takes niacin, you can see their face get red, and they will itch. For me, my scalp usually itches the most. The flush lasts a few minutes.
It does not do anything, in my opinion, for blood flow in the penis.
The flushing tends to only last a max of 30 min. I have found that if you are drinking alot of water the flushing tends to be less severe. Also if you step outside around the time the flush begins (assuming its chilly outside) that will also help the keep the flushing more tolerable.