The use of an engine after the Preparatory signal is not allowed. However JOG General Condition 14c allows its use in a specific circumstance:
A Yacht which is required by the Sailing Instructions to pass through an identity gate and does so, for the first time, after her preparatory signal, may use an engine or tow provided that she:- (i) switches off her engine or drops her tow and then completes a 360 degree turn before starting; (ii) does not start until five minutes after her starting signal (iii) reports the incident on her Declaration Form.
(Usually the above allows for any delay in your arrival for the start).