It's pretty simple. You may only post if your user ID is higher or lower (depending on what they choose) than the user above you.
Example:
Bob (User 400):
Higher
Joe (User 5001):
Lower
Billy (User 4024):
Higher
I'll start us off, higher.
Example:
Bob (User 400):
Higher
Joe (User 5001):
Lower
Billy (User 4024):
Higher
I'll start us off, higher.
