Alpha Arbutin works at the source blocking the tyrosinase enzyme that triggers melanin production in the first place. Niacinamide picks up where it leaves off stopping that melanin from travelling to the skin's surface where it becomes a visible dark spot.
Together they attack pigmentation at two separate steps in the same pathway. That is why Petalspursh delivers faster, more visible correction than any single active brightening serum because two mechanisms working in sequence will always outperform one working alone.