i've totally seen somebody like 1 year ago, doing the loved up walk animation while holding a noxious scythe. I have a cat staff and can't do the loved up animation with holding it. this makes me feel a little sad-ish-ish. sorry im only like not even 2nd grade on anything of education/knowledge/IQ
I think it would be technically considered a graphical bug. To try and recreate it, I'd suggest trying activating and deactivating the walk, 2h staff overrides, sheathing and unsheathing your weapon, resting and un-resting, praying and un-praying, etc. That's usually how such bugs are achieved.
To reiterate: it isn't supposed to work that way. They just figured out a way to make it happen and then didn't change anything.
For a while you could use some rests like the contact juggling rest while holding items, which looked really bizarre. It was done through manipulation of animations and timings using prayer hotkeys.