I had a LOT of concerns about this issue so I called myself and here is the information that I was given:
When you check in, there is a hold for $1 placed on your credit card. There is nothing held other than that. (Unless you are paying your room balance...that is charged upon check in)
They then give you the option of charging things to your room. You do NOT have to do this if you don't want to! The way it then works is, they open a line of credit. The MAXIMUM amount you can open is:
$500 for the value resorts
$1000 for the moderate resorts
$1500 for the deluxe resorts
Remember that that is only the maximum. You can specify any amount UP to the amounts listed. Also, they do NOT put a hold on the amount.
As you charge your way around the world, the purchases are billed to your room. When you accrue purchases in the amount of your credit line, it is THEN billed to your room, and a new line of credit is opened automatically. If you don't accrue up to that credit line, I think the charges are put through to your credit card when you check out.