Hi –
I found this post,
https://blog.cgcookie.com/forums/topic/cant-find-uvs-on-truck/
“Here’s a quick explanation of what blender does when copying and pasting poses …
If a bone has a “.L” suffix and you copy to the buffer and then paste mirrored. It will attempt to find the “.R” bone and flip the relevant channels and paste it to that bone.”
I watched the video again several times and was confused too. The class is using version 2.78 and with that version he does copy the foot_IK.R but when he mirrors it blender automatically copies it to foot_IK.L. So, he isn’t mirroring the pose to foot_IK.R. I use version 2.90 and it doesn’t work like that for me, I need to select the bone and past flipped. So you do understand it right. Hope this helped.