Incise Flow computes the number of cells upstream flow at each point, then does a logarithm on that count and subtracts it from the height. The flow map is computed by moving from the current cell towards the highest cell in the 8-way neighborhood, adding to a count at each point. This means that there is an upstream flow journey from each point on the surface. The results aren't quite correct, but are generally good enough.
A search for the term "precipiton" (note spelling) will get you the idea, but it's pretty much pick a random spot on the surface, locate the lowest neighbor, and move some percent of the difference between the two to the lower cell. Then move to the lowest point and keep going until you hit a pit or go out of the area of interest. This model is a simple cellular automaton, unlike Incise Flow.