When you are charged depends on the pass or payment type you have chosen:
Pay as you ride
When you choose "Pay as you ride", you pay for using the bike at the end of your ride. The duration of your ride is counted from when the bike is unlocked to when the bike is locked again. At the end of your ride, you'll be charged a single payment to cover the unlock fee, the number of minutes you were riding the bike and any applicable "Out of Bay" or "Out of zone" fees.
When you buy a minute bundle, you pay for the time you ride up front. Minutes are then deducted from your balance as you ride.
You'll be charged any applicable "Out of Bay" or "Out of zone" fees at the end of your ride.
When you buy a Day Pass you pay upfront. The Day Pass is valid as soon as your purchase it for 24 hours, so don't buy it before you're ready to ride a bike. With a Day Pass you won't be charged any "Out of Bay" or "Out of zone" fees during the 24 hour period. As long as you leave the bike in a Bay at the end of the 24 hour period you won't be charged any additional fees.