|Green Pirates Shaft||Adjacent rooms||Parlor and Alcatraz|
This room was named after the most common strat, which was found by 4n6, that never stops moving and shooting. Some runners later noted that the name also fits because you visit this room twice in most runs, hence "I'll be back."
There are several strats for this room; the most common involves entering the room with full run speed, and holding the Shot and Angle Down buttons though the door transition. The rhythm of the uncharged Power Beam shots happens to kill the Geemers on the way down. Then after the third Geemer killed this way, fire a missile as you reach the last Mushroom to kill the first Waver, along with a Power Beam shot that should hit the door and open it. Then shortly after you collect the Energy Tank, fire another missile at the second Waver. You should be able to pick up the missile pickup from the first Waver after firing this missile, so you should be able to leave the room with five missiles.
There are other viable strategies for this room, several of which are either faster (though having more arm pumping) or more consistent. For instance, this strat involves bonking the door on the way into this room, which means the run speed is guaranteed to be consistent.
Beginners can forgo the missiles on the Wavers and simply take the 10 damage from running through them. Given that it's far more important to have full missiles than full health going into Brinstar for Any% KPDR, the typical route runs through the pirates in the next room for 60 damage anyway. If you let go of Angle Down after killing the third Geemer and keep holding shot, one of your shots will open the door.
Another alternative strat is to jump over the Wavers after collecting the Energy Tank, and door check the transition. Since the Wavers keep moving during the fade out, one of them will collide with Samus. During the door transition, hold Jump and Left, and you can damage boost past the top platform, landing on the second platform. This will allow you to get through the first Pirate before your i-frames run out, reducing the damage you'll take in that room. This will increase the time taken by the door transition, but could decrease the time of the next room enough to be worth it compared with the beginner's strategy outlined above.
Most categories will come through this room later on, on the way to Tourian. If you have Speed Booster and Wave Beam, there are several strats available, as seen in this video:
The speedball is the most common strat for high-level runners; it fixes the door, while also conserving momentum going into Green Pirates Shaft. At the end of the platform, you can Flatley Unmorph off the ledge to save time in that room.
The Super Missile strat is the fastest for in-game-time, but may not be worth it even in that run due to needing to conserve Supers for Tourian.
While the beam strat is slightly faster than the strat of jumping through the last Waver with blue suit, for categories that have Ice Beam equipped it does run the risk of accidentally freezing an enemy in your path if you time the shot poorly. As a result runners who aren't consistent with the speedball may find the blue suit strategy to be the best for them.
The optimal strat involves a short-hop speedball, but it is significantly more difficult. As seen in this video, it's possible to do this with either a missile, or with a Plasma beam shot.
This is technically possible with Spazer, but the Wavers make the window for shooting extremely small, and then the jump for the mockball also needs to be only a few frames.