Smoke here is a picture of the Traeger firepot. You can see the igniter glowing red hot as pellets are entering the firepot from the auger. At the same time there are holes in the firepot sides that are forcing air(oxygen) into the firepot causing a vortex from a fan. This is what causes the pellets to burn at very high temperatures...
So once the pellets are ignited there is enough heat and air to cause any additional pellets added to ignite through the cook until you run out of pellets or shut it off.
The Traeger digital controller will add 15 seconds of pellets every minute or so when the temp is above what it is set at to keep the flame from going out. The 3 speed controller is simply that. Three different timing feeds creating a low, medium, and high setting.
Smoke is created as the pellets start to cool and smolder then more pellets are added and ignite... Then the process repeats.