"moving" a keyframe along a the timeline with a slider

I've seen some tutorials involving the key.index function, but they're a bit out of reach for my understanding. I hope someone can guide me to the simple "step one" of it all.

The basic idea, simplified for learning sake.

I've got a shape. I want 2 keyframes on it's position, a fixed number of frames apart from each other.

The positions aren't to be affected by a slider (or anything), those are fixed with some easing.

I want a slider to dictate the time both these keyframes "do their thing". (the second one would be +10 frames after the first one, on the same slider, I assume. One slider for both variables)

I've got the shape, and a null. 

I'm having trouble figuring out where the actual keyframes exist. The shape? The Null? 

And I can't wrap my intermediate head around which layer looks at which keyframes and where the expressions exist. 

Any help would be appreciated, including where to learn more about they key.time/key.index/key.value expressoins.

- another way I'm thinking about it - could I have a single layer with "randomly placed" keyframes, and then a bunch of expressions that "set" the time/value of each of those keyframes, regardless of where I put them to start?