I use both Shimano and Sram chains and I use Sram quick links on all.

Shifting under load will break a chain. I actually used to be able to break chains on demand as a demonstration for customers, and all I did was shift into the small chainring, hold the rear brake lightly while I shifted back and forth a few times on the rear while pedalling. I had 100% success rate for causing failures.

Edit: I just noticed the OP’s handle is ‘Big_Rider’… I, too, am a ‘big rider,’ and that is why I was able to break chains so consistently. Sorry, big rider, but you are going to have to relax your massive leg muscles when shifting from now on… don’t stop pedalling, just don’t shift while exerting 100% power!