Stuff with HD fade in then fade out, stuff without HD just fade in, but both need to be hit within the same amount of time from when they first start to fade in, and both will start to fade in at the same time if their timings are the same. For HD, hitobjects have to fade in faster for the time to fade out and be hit to fit the same interval as non HD. Therefore, a hitobject with HD is faded in sooner than an hitobject without HD. That means the player is able to see the hitobject with HD sooner and react to it sooner.
Tr3sLeches wrote:Why is HR and DT worth more Reaction points than HDHR and HDDT, respectively?