Onion skin enables you to see several successive frames, before and after current frame so you’ll have an idea of how an animation is changing. When animating we often need to refer to previous state, for example, we make the jump action, landing on both feet after take-off, the position where the feet must be in a standing position with the same level initially, as a result, onion skin function is crucial . With onion skin function, we can also make adjustments and changes the movement, while using animation curve, onion skin function can also provide a more intuitive view of the changing process.
Back to the "footprint" of this animation, does the transition feels somewhat stiff when you play the animation? You can use, onion skin feature can help you find the "sweet point"
The move to frame and select "onion skin", select the amount of frame you want to compare(7 for example), the canvas will be displayed on the front seven to the current state, so you can have a glance of the whole sequence. Base on this, adjust the keyframe to make the animation feel smooth.