Panasonic GH5 & Olympus 12-100mm - Manually Focused This Time

Suggested by another YouTube user. Zoom all the way in, set to manual focus and then acquire focus on the subject. Zoom back out (if you want to start recording zoomed out first), then start recording. Once I zoom all the way in it's still in focus and no hunting along the way. I do see a 'shimmer' when I start to zoom while on the wide end and then another shimmer as I'm zooming back out to wide, just before the widest focal length I see a shimmer again. Maybe the IS? Anyone know what that is?