In FSRS, how do your review grades change D and S?

R decays on its own with time; your grade only moves D and S, and it moves them in intuitive directions.

• Again (forgot) → S drops (memory proved less stable than assumed) and D rises (card is harder than thought).
• Good (recalled) → S rises (durability confirmed and extended).
• Easy (trivial) → big S rise and D falls (this card is easier than assumed).

So difficulty and stability are tracked separately and updated by evidence, letting FSRS escape "ease hell": a card you now find easy can have both its stability climb and its difficulty fall.