I think this feature used to exist, I recall a thing where you could specify a multiplier to apply to the force intensity over each frame until it reaches 0. Instant/permanent were just suggestions for 0 and 1, since multiplying by 0 ends the force on the same frame, and multiplying by 1 makes it go on forever since it doesn’t change anything. That also allowed to make not only slowly decreasing forces but slowly increasing ones too.