Backdoor Flash Shift (Bomb Over Shutters) is a technique used to gain access to the Flash Shift room by bomb jumping over the shutter platforms from the other side. Samus can hover over the shutter platform by infinitely bomb jumping, and then bomb jump through after waiting for the shutter door to open.
If Samus touches the shutter platform on the ground, or, if she gets too close to where the shutter closes, then the shutter will not open and remain closed. However, bomb jumping at specific angles won't be detected by the door's shutter.
The two main ways of bombing over shutters is to either do a vertical method or a diagonal method. The diagonal one is usually more forgiving and more lenient to perform on the timings. Both techniques require laying an upper bomb and a lower bomb, usually starting with the lower one.
Your bomb jumps should follow a beat of about 150 beats per minute (BPM). See the Resources section below for a list of songs that approximate the rhythm you need to perform this trick successfully.
For the diagonal method, you want to bomb jump between the corner pocket above the shutter (which will be your upper bomb) and back down close to the center of the shutter platform (which will be your lower bomb).
The key to getting the diagonal method right is being fast with your left and right movements, holding each direction nearly the entire time before you catch your latest explosion, and also the timing of your lower bomb. Try and keep the placing of your lower bomb consistent, it helps a lot, and adjusting the timing of your lower bomb will decide whether you stay in the air long enough for the shutter to open.
For the diagonal method, you want to position yourself off-center of the platform, closer to where the shutter door is.
Start a bomb jump on the ground, or in the air, slightly above the platform.
Hold right immediately to move diagonally into the pocket, then lay your upper bomb, then immediately after hold back left to catch your lower bomb.
Lay your new lower bomb in the air above where you started, and then immediately hold right again to catch your last upper bomb.
Repeat these last two steps until the shutter door opens.
If done correctly, you will alternate between bombing in the corner pocket just above and to the left of the shutter door, and between the near-center of the shutter platform where you started.
The vertical method is much harder to pull off consistently, but with practice, you should be able to make it consistent.
For the vertical method, you want to keep Samus close to centered on the shutter platform,bomb jumping between the ceiling and just above the shutter platform on the ground, approximately around a Morph Ball's height off the ground. Bomb jumping too close to the shutter door will cause it to remain closed, so that is why staying closer to the center is more consistent.
The key to getting the vertical method right is keeping both your lower and upper bomb timings consistent. The bomb timings can be unforgiving since Samus will be bonking into the ceiling in ball form each time.
For the vertical method, position yourself close to the center of the platform, away from where the shutter door is. If you are too close, the door will sense you and remain closed.
Start a bomb jump on the ground, or in the air, slightly above the platform.
At the top of the jump (when? before or after hitting the ceiling?), lay your upper bomb. Fall back down and lay your lower bomb right as your last lower bomb is exploding.
Repeat this last step until the shutter door opens.
If done correctly, you will hover vertically long enough for the shutter platform door to open.