How do you (or who does) quantify a "a fair share of the profits based on their contribution" ?
It’s based on a bounty system for tasks, each of which have a value. With value localised to every project, the value can be a function of anything (though likely time and difficulty) as long as it’s consistent throughout the project’s life. When people are content with the amount of value a task will reward, they can begin working on it and as they complete these tasks and they’re peer reviewed and accepted, their value in the project grows.
Hope that goes some way to explaining it. Let me know if it needs more clarification.