From the 1988 Badge Book:

Every guide is expected to do a good turn each day and to help other people whenever they can.  The Service Flash is awarded for doing extra service.  It should be a service which is needed and not just something you would like to do.  If the work is still needed when you have completed the required 24 hours, you must continue or find another guide to take over from you so that the people you are helping are not let down.  It will take time, so if you are already too busy to do more you should not attempt the Service Flash.  The Flash is not awarded for service already being done or for anything which is your duty to do eg it must not be for your family as it is your duty to help them.  Before commencing this Flash you must discuss the choice of service with your district commissioner and obtain her approval of its suitability.

The Test

Carry out within a six month period a minimum of 24 hour service to a group or individual that will provide you with a challenging experience.

Note:  The Flash can be awarded following completion of 24 hours service but it is expected that you will continue this service if it is needed.