We see Ransom stabbing Marta in the chest, but there was some weird noise, and apparently the kill didn’t work. What did happen? Did the knife retract? I have no clue.
2They demonstrate the retractable feature of the knife immediately after the initial attempt. -1. youtu.be/fLkBDy8ZMy0?t=164– CharlesMar 2, 2020 at 12:15
1In the movie, it is shown pretty clearly that it is retractable.– SujitMar 2, 2020 at 12:22
Yes, it's a fake knife.
Such knives have a retractable blade, which apparently is not sharp. It is generally harmless and used in party tricks.
If you recall from earlier in the film, Harlan criticizes Ransom's protected lifestyle saying he is unable to tell the difference between a stage prop and a real knife. That was a bit of foreshadowing, as Ransom grabs a knife to kill Marta, only to find out she hasn't died. That's because, as Harlan foreshadowed, Ransom didn't know the difference.