Assist feeding is something you do after you've tried EVERYTHING you can think of or if your snake is loosing a significant amount of weight.
Essentially, you take a f/t rodent and use its nose to gently pry open the snakes mouth. Once the mouths open, you slide the rodent in a bit, catch the rodent on the snakes teeth and set it back down in the cage. It may take a few tries as they may spit it out, that's why you make sure the rodent is hooked on the snakes teeth. You do not shove the rodent down its throat.