Solution to an issue with mirroring
Not a question but I'm sharing a fix to an issue I struggled a while to solve:
Issue : flip-pasting from 'foot_IK.R' to 'foot_IK.L' (as shown beginning at 3:45 in the video) was working but as soon as I moved away from the current frame 'foot_IK.L' was reversing to where it was before the flip-paste.
Fix : set the 'Active Keying Set' in the Timeline window to 'Available' (mine was set to something else).
I thought it might help others (and a note to myself : study a bit closer how exactly these 'keying sets' work !)