Normal rashes caused by increased bloodflow to an area will turn white when the tumbler is pressed on it. This is because the pressure of the tumbler forces the blood out of the tissues. The rash from septiceamic meningitis is caused by ruptured capillaries, leaking blood under the skin. This cannot be forced back out under pressure and so will not disappear.
sorry W, I dont know why, but I do know this also happens if you stretch the skin with you hands (if you dont have a glass handy). Just a useful bit of info...!