Here's one thing I don't understand, though. I thought that on the secondary side of a transformer, the vintage truly is floating, and it really only wants to flow from one secondary lead to the other, or to the center tap, if it's rectified. So why would touching a single lead even "bite" me at all? Where's the reference voltage that says "your finger is at a much lower voltage than the wire, therefore, the wire's voltage will flow through your finger to reach ground". Why is it not just that nothing happens, since your finger is now referenced to the secondary voltage?
Once you ground the HT CT to the chassis it is no longer floating. Current goes through you, out your shoes, through ground, back up the safety ground, into the chassis and finally into the HT secondary. That’s if you aren’t touching the chassis or other HT lead with your other hand… and this is why rubber shoes and mat are suggested.
In the case of a transformer not grounded to anything, you are correct. Nothing happens unless you touch 2 leads simultaneously. I think this is what’s happening with the