The Flow Channel

The 'flow channel' is an element commonly used in games design, and as explained by (Sala, 2013). This concept can be used to keep the player engaged in a game and keeps them playing, as it's a system a game can use throughout it to keep the player just at the right level of engagement, without them getting bored and wandering off. You want to have just the right level when making a game as you keep a steady balance with the player's emotional state. To keep this in mind when making a game, you don't want to include long patches of time with the character running forward doing nothing, as that's just boring to play. Likewise, if you have constant action happening on-screen with endless enemies and action, that can also get a bit strenuous to play and the player may just give up to take a break. It's a level designer's job to keep a balance in the game and ensure an equilibrium between the two sides is met, peaking at just the amount of intense action and peace to keep the player the happiest, this can be referred to as a 'flow channel wave'. 

As talked about by (Sala a, 2011), on a separate site to the previous source, anxiety comes into play for a gamer and.it's a developer's job to ensure that they don't go overboard with things that may unnerve the player (unless your game's aimed at a niche market with that exact intent). This is especially true when playing a horror game that's designed to be suspenseful to keep you on edge, some people enjoy it but others don't. If a player becomes both bored of the gameplay and anxious at the same time, then they will get incredibly frustrated instead, at that point the flow channel suggests they will stop playing the game and never touch it again. A game like 'Five Nights at Freddy's' is a good example of a game which usually makes the player just too frustrated to play it. As it essentially involves your waiting in a small room, completely immobile, hoping you don't get jumpscared/killed before the time limit expires or else you'll have to start all over again. A horror game that has a good level of anxiety however is Amnesia, considering it tasks the player with exploring and solving physics based puzzles, but there are a few enemies scattered around which cannot be killed and must be avoid. The fact that enemies are so scarce actually frightens the player more, as it means you don't get used to them constantly arriving, and when they do you must frantically find a way to escape without getting killed. The game really makes the most of the tension before you encounter an enemy, and that's what gives it such a balanced flow channel: causing you to spend more time being scared of an enemy than actually encountering one.

Here's a graph sourced from (Cone, 2013), which agrees with the previous source on balancing the player's emotions to prevent frustration, and this chart shows a good balance between anxiety and boredom using different challenges that require varying amounts of player skill. It is recommended you include an amount similar to this graph in a game as it plays along, allowing it to be enjoyed without stressing the player too much and while also including a good level of endogenous value:

The wiggly line inside the white borders represents the player's emotional state as they play through a game and its difficulty  curve ramps up, and if it strays either too far up or down, they'll end up either too anxious or bored of your game and will likely not want to play it anymore.