From 28 August you will be able to: 

Set the bidding value to a fixed amount, which, if it is the highest, you will auction the domain for. The other option remains to set a limit where the system bids for you. 

Set your limit for a specific auction before it starts. At the time the auction starts, all set limits will be compared and the current lead in the auction will be assigned to the one with the highest or earliest set limit (in case of a match).