This monetization vector is reinforced by the following emotions: mastery (positive) and frustration (negative). By carefully increasing the challenges presented by your in-game tasks (but not the actual complexity of your game mechanics), you're able to create friction, that is, pre-planned reduction in a player's rate of progression that is designed to encourage players to spend money.
Friction can be introduced in the form for difficulty spikes (sections that are much harder than their surroundings) or by steeply increasing the overall difficulty curve at a rate faster than a player's growth in power. Difficulty spikes and rises in difficulty curves are not mutually exclusive, and you can combine both solutions.
Once you settle ...
