Dragging curve questions

1. Dragging up/down increases/decreases the brightness/darkness, though what is happening with dragging left/right on the curve?

2. I am dragging down on the midtown of a curve to match a value in my Info panel from a target on another layer and I drag all the way down and there is no further room to drag down so I need to move up a bit and drag to the right until the value matches. Is this the correct way of doing this?

Thanks.