HDH stands for Head Direct Hit. This is a bullet that fires straight up, and yet never misses to aim at a target's centrepoint from above, which typically means it will hit most heads of the target if fired from the front, or at least hitting Kongou's pipes or Gboro-Gboro's back fin, among other things.
Step by step what happens:
1. Fires a Deco Laser (a shot that doesn't hurt or collide with entities except solid surfaces) straight up
2. Upon 1's fading, shoots a Normal Rapid Round from where 1 fades (it costs 0 OP) to extend its height further.
3. Upon 2's fading, shoots a stationary orb that aims at foes where 2 fades at, pointed 30 degrees below so it's looking down (120 degrees down). This disappears after around 0.5 seconds.
4. After 0.2 seconds upon creation of the orb, the orb aims at the target to shoot a sniper round that strengthens over distance (which is why it's shot upwards for maximum damage). Due to the involvement of a sniper round to hit at a target,
it never misses.5. Shoots a basic M damage shot at point blank when the sniper round hits.
6. Creates a damaging M orb that clings on the target when 5 hits. This in effect automatically sticks to the target for around 0.5 seconds.
7. Shoots a basic SS damage shot at point blank after 0.2 seconds of the orb spawning (it doesn't have to cling to actually fire something, but shots will be fired wherever it is at)
8. Shoots a basic M damage shot at point blank when the SS damage shot hits.