Again what ever works is the logical answer.
In reality however...
I have been on all sides of this question; corner marshal, driver & passenger on the ground.
First off, passenger = waving yellow prior to the incident to notify racers of an incident. Well I suppose it could be debris
Second, inform the affected driver of a problem. Ideally it would be a black flag, come to a controlled safe stop off the line. Since corner marshals don't have a black flag the next best thing is a mechanical wave off with the debris flag.
Remember here are the priorities for all marshals in order: 1) Protect yourself & partner(s). 2) Protect the riders still racing. 3) Protect the downed racers.
This is the job of the marshals. If they need to change flags to do this that is part of the job.
No different than as a driver to avoid contact with other drivers.