XMM scheduling supports pilot/follow-up observations, which work as follows:
A success criterion is defined for a (normally short) first pilot observation of a target. If the pilot observation fails, no long follow-up observation will follow and XMM will continue observing according to the original schedule (which is developed based on the assumption that the pilot observation will fail). If, on the other hand, the success criterion of the pilot observation is met, then the XMM observing schedule will be manually interrupted and new commands, with the follow-up observation following the pilot as fast as possible, will be linked up to the satellite.
One example for such a pilot/follow-up observation could be an attempt to observe a strongly variable source only when it is in its ``on'' state.