every 5 minutes your bandwidth is measured.
at the end of the month all measurements are summed up and divided by the amount of measurements. Then the top 5% is removed and you get charged for the other 95%.
So if you have incidental peaks you wont have to pay for those.
5% equals about 36 hours per month, so if you pull big bandwidth over that period it will reflect your statement in the end though.
Hope that helps.
See also;
http://en.wikipedia.org/wiki/Burstable_billing
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