In the Railgun anime's third season in episode 6 at 19:36, Baba orders a type-Mantis robot to seize Misaka, but as the mantis claw (pretty sure real mantises don't have claws like that- but I digress) closes in, the screen blinks white just for an instant (as if to say that moment is too cool or happens to fast for the viewer to see), and Misaka somehow severs the claw and some of the arm it is attached to, launching it into the sky. One sees as the claw arm flies up and when it lands that the separation point looks like a clean cut, and is red-hot. The screen then shows the mantis looking at what's left of the arm it tried to grab her with- also showing what looks like a red-hot, clean cut, where the red-hot quality extends what looks like at least one hand-breadth into the length of the arm where the separation happened.
Again, the screen didn't show how it happened. It showed the instants before and after it happened. Misaka herself is shown standing completely stationary right up until the moment of "impact" where the screen momentarily whites out, and again when she is back in the shot (perhaps to call to mind something like quick-draw sword combat).
This puzzled me. How did she do it? Which of her abilities did she use to do it?
She's not shown holding any weapon, and she's wearing gym clothes for the Daihaseisai festival- I don't see how she could conceal one on her body capable of doing that.
To my memory, having watched the three seasons of the anime before, and now being in the process of watching them again, she is never shown doing something similar or creating a similar effect/outcome (red-hot, clean cut of metal), so as someone who hasn't read the light novels or their manga adaptation, I also wonder if this is something that only happens in the anime, or if it also happens in the light novels and their manga adaptation, and if so- whether there's any more information there or in any other moments that explains how she did this. My memory isn't perfect, so maybe I've just overlooked or forgotten some other similar, clearer moment.
I know she can make a sword out of ironsand (season 1 ep4 at 18:12), but there's no trace of her using ironsand in this scene, and I'm also a little doubtful that her ironsand sword would cause more than one hand-breadth of red heat to linger in the severed arm. Giving a closer look at the scene, 19:16 does seem to show some specks of ironsand materializing out of seemingly nowhere / her hand, but that does seem awfully subtle.
As part of my research, I also tried to think of other scenarios in the anime where the opportunity was there to show her using a similar ability. In season one, there are two scenes against Therestina where Therestina has Misaka in the clutches of her robotic suit (ep23 at 16:04, and ep24 at 16:18), but in both those scenes, Misaka is mostly incapacitated by Capacity Down. In season two, there's the scene where she launches some steel beams at Accelerator and he reflects them back to her (ep6 at 3:54), but I don't see how such an ability to cut through metal would save her there, unless cutting the beams could divert the split sides away from impact with her body (which I find physically hard to imagine at the speeds they were travelling at), and there's a short kerfuffle with a powered suit (ep20 at 11:27), but in that fight scene, Misaka either dodged the suit's physical attacks with its blunt weapon, or blocked them using a moving stream of ironsand as a shield.