IB will attempt to route hidden orders to exchanges that support the order type natively. If there are no exchanges available with native support of hidden orders, IB may simulate a hidden order by holding the order and submitting an Immediate or Cancel order to an exchange when the order becomes marketable.
Although IB will attempt to handle a hidden order in the same way that it would be handled natively on an exchange, there exists a remote possibility that an execution will be delayed or may not take place. Execution of hidden orders is on a best-efforts basis.