That's actually not completelly accurate. The study protocol (https://osf.io/hkv8p https://osf.io/hkv8p) describes the procedure in greater detail. People were

That's actually not completelly accurate. The study protocol (https://osf.io/hkv8p https://osf.io/hkv8p) describes the procedure in greater detail. People were pressing one button for heads and another button for heads (which we deemend less error prone and less likely to be subcontiously influenced). The trick was that the next coin flip started the same side-up as the previous landed. Therefore there was no need to record the start (and we randomized the starting position of every 100th flip) We also did some auditing of the video recordings (trying to decode the outcomes from the videos) and they showed quite consistent degree of bias as the original responses.