1. Mountain terrain is not level.
Although this is sort of a no-brainer, you'd be surprised how many snowmobilers struggle in the mountains because of balance. You see, here's the problem: when you're at lower elevations, it's easier to distinguish between up and down or flat and not flat. But in the mountains, since there's seldom any flat, things that look down are not always down-there's a bunch of convoluted angles mashed together. You may think you're on the uphill side of your sled because of an optical illusion and distorted perceptions.
2. Counter-steering. how it works.
It's easy to understand how the snowmobile steers on the trail-everything is basically controlled by the carbides or wear bars on the skis. But in the powder, the wear bar has much less influence than the actual flat surface of the ski. So here's a good way to see what steering/counter-steering is doing. Hold your hand out straight in front of you, palm down and fingers together and pretend this is your ski (your arm is your snowmobile). Now to simulate the deep powder effect on your snowmobile, push your palm down so the angle becomes about 30-45 degrees (fingers pointing up) and turn your palm to the left. You can see that the flat part of your skis is now pushing into the snow and will likely drift to the right. That's counter-steering. The flat part becomes a sail and literally is pushed opposite the direction it faces.