That is right this video is not about Split Ring!
This was also my query and I could not find any answer on the web, so had to think and create my one.
My splint ring was about 15mm in diameter and it was opened almost like a spring. I'd say with a gap of about 5 mm .
I tried to grab both ends from the opposite entry (inner ring) to pull them toward each other but this did not work.
But my success came when I leveraged open ends (one at the time) inside the ring. First with long nose ordinary pliers. Then found out that a good quality sharp pointy pocket knife was a much better tool for this job.
You don't have to start with the ends as any part of each ring can be levered inwards below the other opposite ring.
Then it was just a matter of reshaping a little the rings till you get your circle okay. I did it all with my long nose pliers and it took me 5 or 10 minutes at most to do it once I knew how to go about it.
I think the splint ring gauge was about 0.5~1mm . I was very happy with the result and guess with more of the same work it would have been perfect! But as it was it was good enough with both rings in contact. (I use the term 'both rings' but hope you understand it is only one but with both end forming an opposite ring.)
I also realise this site is more about leather work but since the question was there I wanted to help.
PS I always forget to notice the date of the post...2013! That is a long time ago I wonder if there would be a more appropriate place to have my finding?