I loosened the logic board attached to the power supply board. I then could easily remove the 2 power cables.
Upgraded my iMac 21.5 inches Late 2012 to 16 GB using the iFixit Pizza-Roller. It worked fine. Absolutely important not to pinch the tool.
The only time I had to modify the procedure was at step 42. I could not reach the power connectors so I left them in place. After removing the logic board I could disconnect the power supply without a problem. Reinstalling the power supply is a little tricky as well.
I had left the iMac overnight with the power cable disconnected and the display loosened, just slightly opened. This asserted me that there was no charge left and that the glued stripes had time to dry up. The residue stripes were easy to remove.
Finally it saved me a lot of money as I got the RAM-modules for just 143€ - compared to an online offer of 380 US$ from Apple for the whole procedure, including the RAM modules.
Thank you iFixit for that great video.