I'm not into the numbers of how fast something goes vs the shutter speed and whether it will accurately stop the motion.

From experience, however, I have used fast shutter speeds for lots of sports with no visible motion blur.

While in Egypt I took a shot of Hosni Mubarak's helicopter as it circled over before landing. At 1/4000th of a second the rotor blades looked perfectly stopped. Maybe if it was enlarged a small amount of movement might be perceptable.

In the photo it was cool to see a huge copter stopped and "hanging" in the air.

"So it seems that higher shutter speed is a mechanism for controlling the amount of light that gets onto each photosite of the sensor to control overall exposure settings, but it won't work for freezing motion."

No we did not move into discussing HSS just to argue. The above part of the OP postulates that a fast Shutter won't "work for freezing motion" and then asks if he is right or wrong. That was answered in about a second (he was wrong).

Then we went off to try and explain that his understanding of 1st and 2nd curtain and how "mechanically" it worked trying to pass along some information that was absolutely pertinent to the topic if you want to begin a serious discussion about how the Shutter freezes motion. HSS came into it because some reply's thought that a strobe was the thing that froze the motion. The difference between the Eye and the Shutter came in because some didn't understand how the Flash worked with HSS (because it worked for their eye to see fast things). The OP was basically correct about the Shutter controlling the amount of light that reaches the Sensor but the topic (some of us felt) could stand a little expanding. My original reply answered the question that was asked and then tried to quantify my reply, the resulting frenzy occurred when certain other people disagreed with some of my point and we discussed them, BIG DEAL.

Like I said in my earlier reply after some people started to basically argue with what is generally an accepted fact (that a high speed Shutter does stop motion), if we cannot expand on a topic and hear from those that might be more informed why have a forum? Is this site like a court where certain "facts" are not allowed because they were not part of this exact topic? Even though they might just take the topic in a direction where people can pickup some good information? Then there are those that feel it necessary to post a reply about people that posted a reply so they can accomplish what? And within their "sensible" reply they draw the same conclusion we did but provide nothing additional. Seems to me those are the people that just want to bicker.

I actually thought that by exploring the topic of freezing motion in its entirety we might have some really cool input from people but that is far from what I got. So I apologize for trying to go too deep and trust me it won't happen again. I have to prepare for a big shoot now and It just might involve a fast Shutter so wish me luck.

After much digging I did discover that I was wrong, somewhat anyway. Yes there are 2 curtains, one opening, the other closing. My deepest apologies, but yet, my sincere thanks as well. It seems that I learned something again tonight. Now I can go rest. Now, back to the original question; what was it again?