in a muscle growth drive, a person creates a small character to grow, people donate money to add pounds to the character, and after the artist reaches a goal they make an update until the final amount is reached

Growth drive is like a fund raising thing.
Steps.
1. Advertise your growth drive, and the (valid) reason for gaining the money.
2. On the advertising show the "stage 0" of your character of choice, usually with lean/twink/little body build.
3. Once you hit the first fraction of your goal post the "stage 1" of mentioned character, by such some musculature have been added.
4. The more fractions of your goal has been completed the more progressive the muscle gain the character get.
5. Once the goal is reach the mentioned character is not at its full muscular glory.

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Oh right... You're still new here. Well there you go. How most growth drives work.

it's similar to that, and the comments are both right, however they can also be interactive, such as having people be able to put money towards certain parts being grown, such as adding more mass to biceps, thighs, or pecs, as well as having goals that add your own things, like special bonuses. i've even known an artist or two to feature cameos for some if they paid enough to support them. i think most artists that do it have milestone dates, where they check the progress of it and determine how the character will look at that point, and let it continue while they work on it, doing it more as it goes on and releasing the pictures as they finish them. and i think having interaction like that also gives you an insight as to what specific parts your audience enjoys most. i'm sure you'd get lots of pec enthusiasts

In a simple laymen term..

"Growth Drives, are a fund raising system where, a general character which starts off as a slim person, goes through a either a fat,muscle or height growth transformation.. As money is payed the heavier, muscular or taller the person becomes, so if you set it as 2 dollars per lbs that would give the character an extra pound in weight. After a specific amount raised for each picture we see the next stage in what happens, we keep it up until the goal amount is given."

there are also variations on the same system where you can make parts of the body grow rather than all of it.. makes a more random experience.

I think it's when you sell chocolate bars on the street for school

Yeah, uh, you can also do growth drives for free.
If you're interested.
How that works is people comment on the body part they want to grow, or even a person, if the growing is done by more people/rivals hungering to get bigger.

Here, some reference to a free growth drive.
http://www.furaffinity.net/view/15158007/

There are plenty of variations of growth drives around. Based on what I see, the core concept is to have regular art posts, and increase the size of whoever you're drawing based on the amount of something. I think the most common "something" is the amount of cash donated, though it can be votes, comments, or whatever else.