"Pre-authorisation" means a temporary hold of a specific amount on a credit card. This transaction does not generate any type of bank movement on your current account because it is not a charge.
When you send a "payment invitation" from SplittyPay, higher amounts are pre-authorised on the credit card entered to ensure the immediate confirmation of the purchase. If all the people invited pay their shares, the charges will correspond exactly to the amounts you set during the payment phase.
If, on the other hand, some participants don't contribute, their fee will be charged to the other cards entered.