We told people that the coin has to flip at least once (which would bias it for the opposite site). Whenever instructing people, I tried to explaining that the coin flip should look like you were trying to determine an outcome of a bet.
You can find the complete experimental protocol here: https://osf.io/hkv8p
Also, I wish I had (any) budget to hire proffesional skilled tossers haha.
I assumed that the 1% bias was entirely due to coins that did not undergo any rotation at all. However, reading that you told people that the coin has to flip at least once, I think I assumed wrongly. It sounds like the bias is due to coins that have undergone an integral number of 360-degree rotations (not zero rotations). But what exactly is the physical mechanism causing this bias? It's easy to understand why zero rotations would introduce a bias, but I can't easily picture a reason for a bias toward an integral number of 360-degree rotations. Is there a simple and intuitive way you can explain the physical reason?
It's not about the number of rotations at all. I doubt that you can control it at all even after dozens of hours coin flipping (I did more than 20h and I can't eveb guess how many rotations the coin made) Diaconis, Holmes, and Montgomery (2007) proposed a physical model of coin flipping that introduces the bias as a result of wobblines (i.e., off-axis rotation in the flips).
Also, I wish I had (any) budget to hire proffesional skilled tossers haha.