The basics of this OS have been explained before, but I’d like to go into some of its technical details. This is not meant as a how-to, however, understanding the mechanics involved can help you to execute it properly. For those familiar with yeb’s video explanation, note that I’ll explain things a bit differently. The key mechanic that makes the OS work is the disabled “negative edge” release input when it’s preceded by a press. Also, the release needs to be timed on the transitional frames that come after a move’s recovery is over.

What are transitional frames?

I’ll use Cammy’s cr.MP as an example: it has a total of 15 frames, but in addition to that, there are 5 transitional frames at the end of it. These extra frames can be seen in the SSFIV hitbox videos, but since they’re not part of the recovery and can be interrupted, they don’t have much gameplay effect. They’re mainly there to smooth out the animation’s transition back to regular crouching. For a crouching normal like that, in order for the extra 5 frames to play, you need to hold (any) down. For a standing normal, you would need to remain in neutral. This is related to what yeb described in his video as “state changes” that trigger the OS; what’s happening is that when the other character attacks or jumps over Cammy, the post-recovery phase of her cr.MP gets interrupted and she goes into another animation. If Cammy is holding downback during the transitional frames of her cr.MP, she’ll get interrupted if the opponent threatens her guard with an attack. The change in animation – from cr.MP to guard – is what triggers the PBOS. However, a specific input is required so that it only triggers on this condition.

How does “negative edge” get disabled?

If Cammy wants to PBOS an HK Spiral Arrow after her cr.MP, she needs to time the HK release on one of the 5 transitional frames available, but it has to be preceded by an HK press that’s done while still in recovery. Pressing a button during a move* disables its release counterpart for several frames, but only for that specific move. So, as soon as the animation changes, the release input becomes valid and the special is executed. If the HK is held beforehand, the OS won’t work and the release will always result in a special coming out. In order to disable the release input, the maximum gap possible between button press and release is 6 frames. This means that if HK is pressed on frame 1 in a timeline, it needs to be released on frame 8 at the latest.

This doesn’t apply in all situations; the release input doesn’t get disabled during dashing, block/hitstun, wake up, stun recovery… In order for it to get disabled, the move itself needs to be something that gets triggered by a button press. That would include normals, specials, etc.

Which moves are valid for PBOS?

Because a standing normal gets interrupted by backwards movement, only crouching normals are valid for the OS if you wish to trigger it via proximity block. Special moves can also be used, but they need to end in a crouched transition, like Sagat’s Low Tiger Shot for example. It’s important to note that a move with 0 transitional frames can’t be used at all. An example of that would be Sakura’s cr.MP: since it goes straight into the crouching animation after its recovery, there isn’t any frame where you could input the OS. Other examples of such moves would be Guile’s cr.MK, Akuma’s cr.HK and Yun’s cr.MP. Note that most moves in the game have transitional frames, and many of the characters have some in all of their crouching normals. The difficulty of timing the release is affected by the number of transitional frames a move has; it’s easier with more frames, but it’s only meaningful up to 7 frames, because any additional ones beyond that would fall outside the allowable distance from the press. Performing the OS with Ryu’s cr.HP for example would be difficult, since it only has 1 transitional frame, which translates into a 1-frame window for the release.

Where can I find how many transitional frames move X has?

This type of frame data is uncommon and not really useful, which is why it isn’t readily available from anywhere. Unfortunately, I don’t have the time to gather it. I can answer specific questions regarding this frame data, but please keep them to a limit.

If you have any questions about the OS or want something clarified, go ahead.