There's two requirements to do it:

1. Being in combat. You can notice when you're in combat when the HUD "activates" itself when it's hidden. You can also notice it through Leon's animation. The parts where Leon is in combat are fixed points in the game, so you can memorize where and when you can do it.

2. Not having the dufflebag glitch. Some parts of the game you're in a state called "dufflebag", received it's name due to allowing you to carry ashley like a dufflebag in ch15. When you're in combat and in this state, you're slower than usual and Leon does not to the normal in combat animation. Some places where it happens naturally for example is CH2 in the canyon/dynamite valley, where you can not do the crouch dash.

As for how to do it, it's simply crouching and sprinting, as for achieving the proper timing, I recommend watching the latest runs from Missing.

It's extremely easy to downpatch on PC. You can download through steam the old version of the game. There's guides on how to, just follow them.

If you have any mods, disable all of them to make sure it's not it. What version did you downpatch to? If it's release version, remove the re_chunk_000.pak.patch_001.pak from your game folder and it should work.