I believe they are looking for a solution with a high content of H2O2. I suppose the theory is that if the content of H2O2 in the solution is high enough to use the synthesize explosives, the vapors will be sufficient to cause the strip to test positive. I could be way off, but that's my understanding of what this is about. I decline to defend the wisdom of the practice, however.
As I have discussed at some length
in this thread (including an interesting exchange with TSORon), peroxides are not volatile enough, and test strips not sensitive enough, to detect traces of them in the air above a solution, no matter how concentrated.
There are no test strips capable of detecting peroxides when waved above a solution. This is pure security theater.