Before on patch 6.13 http://na.leagueoflegends.com/en/news/game-updates/patch/patch-613-notes It stated that syndra's ultimate will scan for balls during the cast instead of before and recently i have been playing syndra and realised that this is no longer the case.
I know this to be true because i went into the custom tool and tested it out
PICS HERE: https://imgur.com/a/S1Gv4
The ball hasn't formed yet. The ball forms at the end of the delay. The best you can do is Q -> 0.25 second delay -> R
This may be a problem on what you perceive as cast time.
The "on-cast"/"during cast" may be referring to the slight cast time that Syndra has. If you use her ultimate, you should see a, for a very small amount of time on your screen, a bar that fills up with a blue line. That bar represent her cast time. They may be referring to that. Not during her whole ultimate animation or whatever.
-Syndra main
