Let me explain trenching.
The blue area represents the amount of snow the track moves. The sled is moving from right to left in the picture. Each subsequent position represents how the sled travels from "c" to "b" to "a", spending a certain amount of time to get through each blue rectangle of snow. We can actually measure the amount of snow in each rectangle, but more importantly its weight. Because your forward acceleration is proportional to the weight of the snow (as well as the snow's mechanical strength as connected to snow further down).
What happens when you trench is that the speed of the track goes up, so you remove more snow than what you find in the "a" rectangle. So therefore the track has to dig down to find more snow. This increases the amount of snow the track has access to, but the deeper snow is often higher weight per volume as well so even a slight dig down could result in huge increase in forward momentum. The faster you go the more snow you have access to without digging down.
However, if you spin the track just too fast in one moment of time you remove all the snow underneath the track before it has time to move enough forwards to get into new snow.
When you have a good throttle control on a powerful engine you get the top line, you keep up speed so that your track never has to dig down for snow. But most riders simply can't pay attention fast enough in the forest so they slow down regularly to have time to decide, resulting in the next line of events "f" to "d". Where they slow down a bit, then hit the throttle a bit to gain speed, but if the snow is decent density and strength compared to their engine power, track length and lug size, they get on top of the snow again very quickly.
However, in very deep snow with low density and really fluffy snow conditions where the snow does not have any strength to bite onto, especially with short tracks and high power, then such stop-start riding will result in the bottom line of events. Where the track was relatively on top of the snow, but the rider slows down a bit (due to a hill or bump or releasing throttle), and then never manages to remove enough snow to get back on top of the snow.
The best trick to stop trenching is to ride a little bit smoother, so that you don't have to make significant slow-down to think and decide where to go next, because if you braaaap it and then slow, you trench easily. When doing big hills and such you need to work up the speed far earlier than you think to get a smooth increase in speed so that you don't trench before hitting the hill.
