It really is the creepy, is-my-nib-going-to-split-in-two slit responsible for the tines being able to spread. The steel itself looks slightly thicker than the Tachikawa School-G nib, which is the only other modern steel nib I know that supposedly flexes. (No, it doesn’t.) (It’s good for other things though.)
Does it flex? Yes, it does. Does it flex easily? No, it doesn’t. The tip has a bit of tooth. This can make the nib catch on paper and turn a smooth curved stroke into a slightly askew one. I also experienced railroading in the beginning, but that seems to have resolved itself. (It could be a matter of getting the feed properly primed.)
My big realization about the Noodler’s Creaper is while it’s good for writing, it’s amazing for drawing. For artists who have wanted to try vintage flexible nibs but have been put off by the price range and the corresponding risk, the Noodler’s Creaper is a great buy. If you’re tired of your dip pen nibs rusting away in less than a week, this pen performs pretty closely to a Nikko G steel nib. The Nikko G goes from extra fine to medium-broad, the Noodler’s Creaper from fine to extra broad. The video shows the pen in both writing and drawing mode.
I like Noodler’s Black Swan in Australian Roses too. It is a dark sibling of Pilot Yama-budo. When wet with a brush, the diluted ink also looks similar to Yama-budo, becoming a vibrant pink.
Nathan’s original Black Swan in Australian Roses video is of course a lot more detailed. Antonios did a quick test on video of the Noodler’s Creaper nib. Inkophile reviewed both pen and ink, and you can also look at David’s writing sample. Bleubug’s It Ain’t Hip to Be Stiff exposes the innards of the Noodler’s Creaper, and is the most comprehensive review out there.