Twin Shot runs at 25 frames per second. This creates a problem, which is explained in detail in this post, along with a solution.

The problem

Some time ago, it was found out that Flash handles unusual framerates like Red Ball's 31fps (or this game's 25fps) very strangely. For some time, it was believed that it essentially aligns all frames to 60fps frames; so Red Ball, for example, would have most frames be 1/30th of a second (two 60fps frames) while some frames would be 1/60th of a second, even on a theoretical perfect scenario with no lag, which really sucked for a frame-perfects based game like Red Ball (this also affects some strategies in Twin Shot, mainly having to do with the reliability of buffered inputs). Later on, it was figured out that the behavior is a bit more complex than that, but the gist of it is similar.

The solution

As Red Ball is a popular speed game, there was pressure to make a Flash player without those problems, so Maximum took it upon himself to look at the assembly code of the Flash player and eventually found one jump instruction which could be changed so that it jumps over the "terrible code" that creates this problem, and thus, the fixed framerate patch was born. It was accepted pretty much without discussion in the Red Ball community, both due to Maximum's status as a trusted long-time runner of the game, and the community's general acceptance of various modifications of the game before that. It has also been accepted in the Super Mario 63 community after a one-week period of testing whether there would be any weird side effects.

It's not known if there are any other communities that would allow or ban this modified version of Flash; it's more likely that most Flash game communities don't know about it, and for 30fps or 60fps games—which most Flash games probably fall under—the patch isn't really needed anyway.

Verdict

Although the patch is an unofficial modification of the Flash player, the communities of the two most major Flash speed games have accepted it, and we don't know about any precedent of disallowing it, so we have decided to allow its usage in runs of Twin Shot. Of course, runs done on other Flash versions will still be accepted.

The modification should be minor and only affect the frametime stability, although there's little way of telling if there aren't any unnoticed side-effects.

Link to resource

(most of the contents of this post are copied from an explanation dimusnail shared on the Nitrome Speedrunning Discord server)