A bolt on kicks in when your normal data monthly amount is used up.
If your close to your anniversary date don't add a bolt on because you'll lose any unused data in that bolt on when your tariff refreshes.
Bolt ons like your normal tariff allowances do not roll over at the end of the month.
Also if your on a monthly contract and gone over your data you won't be charged any costs but instead be throttled to very slow Internet speeds.
If you at that point add a bolt in and data you've already gone over your normal data amount will be deducted from the bolt on you have just purchased.
So if you've used your 2GB normal allowance and you add a bolt on for 4GB then you'll get that amount minus any data you've already used over the 2GB contract data allowance.
