'if you know how to grind, you dont need this'-What kind of fucked up statement is that. The reason for rounding your edges under your feet is to avoid catching. When you are competing in high profile events such as the USFSO, X Qual, X Games, Huckfest, ect. you dont want to take the chance of a rail being 'sticky' causing you to fall on ur face and ruin ur run.
Let me break this down for you.
Most rail are made out of fairly soft metals, because they're cheaper and easier to weld together. Most, not all, but most ski resorts use this to save costs because the park isnt their main investment. Now when you slide the rail, you want to be smooth across it, but your edges will rub the metal and create friction, which can slow you down. Too much pressure one way or the other, and your ski digs into the rail, creating a small scratch mark in the rail.
Over time, and many people eating shit, or sliding off the side to avoid eating shit put more and more scratches in the rail. These scratches are tiny slits which slowly turn the smooth rail into a piece of metal sandpaper. This is the number 1 cause of 'sticky' rails.
To fix this, the rail must be buffed and polished smooth. At Vail, we use a metal brush that hooks up to a power drill. This smoothens out the scratches, simalar to sanding a piece of wood; Catch my drift here?
SO, to answer the question, rounding your edges under your feet helps, because a round edege will not catch on these scratches, it slides right over them. In time, and enough rail sliding, and your edges will naturally grind down and become rounded and you will slide better.
I simply jump-start this process by using a low grit stone and rounding my edges off manually. I do this ONLY under the foot from the screws in the toepice to the brake in the heel. About 7' for me. Once they are pretty rounded off, rub them down with a cork to get the burrs out. Then sand it down with a piece of metal sandpaper, then cork again to polish. They dont necessarily slide faster, but the overall slide is smoother. This also helos to avoid catching on knots in wooden logs.
I have used this technique on the skis of Jon Kazody, Chris Turpin, David Bird, Derek Finn, Jeremy Weir, JR Martinez, Greg Tusher, Travis Redd, Josh Bryant, and other skiers around the US and Canada. Trust me, it works.
Disatvantage-if you also use your park skis for downhill racing, you may not hold an edge quite as well. Also, Jon Olsson told me that he notices edge loss in the pipe, so he uses different skis for pipe.
If you have any questions, msg me. If enough people are interested, i can make a video showing you the process.
-'Kevin, how ya been?'
'Oh, you know, up and down'-(Kevin, the elevator guy at the hotel I work at.)