QOL.. Quality Of Life tools for Blender!
The FrameFlipper addon is an addition to the timeline bar that allows you to flip between any two keyframes you set without affecting the animation or needing keyframes..
Vey often while staging animation, it's advantageous to be able to flip between arbitrary frames and see instantly the differences without having to scrub.
Either scrub to anywhere on the timeline and click the A or B Buttons to set target frames.
Or ..enter the desired A/B target frame numbers:
And then simply hit the "FLIP" button to flip between them!
Note that this addon is also available as part of the discounted QOL Tools: Full Collection package!