Definition:
Lean UX simplified version of Task Analysis. A visual representation of the steps users go through and the behaviour they adopt in order to complete their goals. The task model is a living document and should be updated when new user needs are uncovered in a Lean UX process.

 
When to use:
In Lean UX each Persona should be associated with a task model.
 

Anatomy of a task model
1. Goal
The thing that people are trying to do.
2. Phases
The broad stages that people go through to complete a goal.
3. Behaviours
The patterns of behaviour people use to make a decision.
4. Tasks
The individual elements that a user has to know or do to facilitate a decision.
5. Flow
Show how people move through the process. Allow for exploration and iteration.
6. Barriers
Identify any pain points.
7. Content requirements
The general and specific content needs that people have around each task.
 

Behaviours in a task model
1. Complex evaluation
A complex evaluation recognises that people need to move freely between often unrelated variables until they reach a decision.
2. Controlled evaluation
In a controlled evaluation people move between known variables in order to make a decision. The interaction should allow people to alter each variable and see the effect on the outcome.
3. Direct connection
In a direct connection people expect to be able to move from one step to the next without deviation.
 

Reference: Richard Caddick
Task Model cheat sheet