The actress is referring to a video published Friday by Daily Mail -- which shows behind-the-scenes footage of Rebel and Sacha filming for 2016's "The Brothers Grimsby" ... and which also appears to show Rebel seemingly being onboard with the whole finger-in-butt thing.

Remember ... Rebel alleged that Sacha had sprung that scene on her out of nowhere while they were filming, but Sacha has said the opposite -- claiming she knew what was up.

Welp, at least based on this clip that DM has obtained -- which purports to show outtake footage that never made it into the film -- it certainly does seem to back up his account ... as you can see Rebel going through the choreography of the scene with Sacha, and she mentions the finger and bum part. On its face, she also doesn't appear distressed.

Despite this ... Rebel is coming out swinging, and strongly indicating these clips are a gross mischaracterization of what really happened -- at least from her POV.

She says, "As I write in my book, SBC is an a**hole. It is an a**hole move to release footage of one part of that disgusting episode while leaving out everything that preceded it including my horrified refusal to stick my finger up SBC’s butt."

RW adds, "This could not have possibly been for the film as the director Louis Leterrier was not even present. What this edited video shows is what I had to do in order to get out of the room, as written in 'Rebel Rising.' Releasing unauthorized and misleading behind the scenes video without my approval is SBC’s latest way of bullying and gaslighting me. The outpouring of messages from so many who have also been mistreated by SBC show I’m not alone."

As we reported ... Rebel has dedicated an entire chapter to Sacha in her new book, which is coming out very soon. Sacha has previously denied her claims calling them "demonstrably false." We've reached out to his team again with her latest claim -- but so far, no word back.