That button enables, or disables the position keeper. The PK is biggest reason why US TDC's were superior to the german ones.
As your undoubtly aware, when you entered information into a german styled TDC in SH3, your info had to be up to date. All information had to be fresh. The TDC did not keep track of the target.
US ones however do. Assuming no change in course, you can enter the speed, range, AOB, and then tell the TDC to track that target or, "keep its position". The analog computer that is the TDC will then, keep track of the target, and increment its exact location at all times, assuming no change in course or speed.
So you can for instance, get one periscope observation, track it with the position keeper, and never have to raise the periscope again - assuming your info is correct, and the target doesnt change course.
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