Traction and forward momentum. When you are on a steep slope and losing both those the track is going to go downhill. The alpha grabs more traction than the others. When I tip my alpha up and hold the ski a foot off the ground, most of the track is on the ground. When I do the same with my gen 4 most of the track is off the ground.
The tmotion is not a gimmick. It does what it is supposed to. Makes the sled easier to ride and ride lighter. Problem is it throws you around a little and you have to work the bars a lot more to keep it going where you want. Doos have always wanted to turn uphill a touch when put on their side and you have to correct them a little. You can put a ski 1 foot in the air and go straight down the trail on the alpha. Recipe for disaster on the gen 4. They automatically want to turn and you have to keep working the bars. Not sure it's all in the tmotion. I think some is in the front. I haven't seen any downfalls with the alpha yet. Probably something. Might just have to ride it different. The gen 4 is great but you have to ride it different than an axys.
Agree with everything you said except for the riding down the trail on one ski. I can do the same on my Doos, make left and right turns as well. But I admit, it DOES take some effort. And you need to have a stiffer front end. (Springs or shocks.)