What's the proper way to use the jake brake

[Roysbigtoys]

Is it more effective to use the jake brake at low or high rpm? Which is better for the engine, low or high rpm?
My best employee tends to leave the jake switch most of the time. If he is shifting up and shifts fast enough the jake being on is not a problem. But there are times when I hear him leave a job site that I hear the jake come on between shifts....drives me crazy. It seems to me that this is not good or am I worrying for nothing?