So with the (e)UXTO model, each operation to change the state is really the creation of a new transaction. So you can think of it as the first contract being restricted as though it was saying "you are allowed to spend the money in this contract as long as you create an identical contract with a balance at least this much higher." With some other restrictions as well. At any moment, the 'creator' of the current version of the contract is the current(and also highest) bid.