We used to use one in my high school days that I would say is the Open/Closed system. We had an indicator, but the coach had to touch the indicator, the sign and back to the indicator for something to be on. If they touched the indicator, the sign and something else, the sign was not on. This is a bit more complex, but it was fairly effective. Teams would think they might have your indicator, but they often didn't think about the closed aspect.
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We used to use one in my high school days that I would say is the Open/Closed system. We had an indicator, but the coach had to touch the indicator, the sign and back to the indicator for something to be on. If they touched the indicator, the sign and something else, the sign was not on. This is a bit more complex, but it was fairly effective. Teams would think they might have your indicator, but they often didn't think about the closed aspect.