If I am correct they are the ones that you put on the tail head in the little patch and they light up and tell you how many times a cow stood during each hour of her heat cycle.
If that is correct- they SUCK! I bought some last year and they don't work at all. My cows had them off in less then a day, they break easy, and just plain don't work. I wouldn't waste any money on them.
We were working calves last week. My 2 sons are really mean to each other! Older son was running the tub and pushing calves up the alley, every time one of my younger son's bull calves came down the alley, the older one shouted "Here comes a candidate for a gomer bull"!!