It seems to be a term for a straight patterned Mangekyou Sharingan, which are synonymous with fluidity of movement. There could be other purposes for the term, but that's all that is known currently.
Actually wasn't Itachi's sharingan better at tracking movement that Sasukes'? Sasuke wasn't able to follow Killer Bee's swords but Itachi was.You must be registered for see images
You will notice that Madara's and Itachi's are curvy, and Kakashi's to an extent as well.
You will also notice that Izuna's, Sasukes and even Shishuis to a certain extent. But they are more straight in design lacking any major curves.
It is my theory that:
Straight Coma Eyes - Are better at tracking movements
Curve Coma Eyes - Are better at tracking Jutsu
I will be making a thread about this soon
Madara is talking about the three commas in a 3T Sharingan.
It's just a try but can you see this? the difference is the alignment of commas in the Sharingan..
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The commas in sasuke's Sharingan is directed straight outside as a whole (don't consider the tail of the comma )
And in Madara's case, it's a little tilted.
By the way, this kind of theory coming from Madara is new so this is all I can say.