Just going to give a quick run down of Flash Sinestro
Start of round is in Sinestros favor b1,j2axe,d2, walk back are all good options
Flash can't really anti-air Sinestro because air axe will beat his d2 everytime if done right (J.I2 air axe) on the other hand Sinestro is great at anti-airing him.
True Sinestros wake-ups aren't the best but Arachnid will get you out of certain situations.
Sinestros d1 has good range along with d2 and b1 Flash has a hard time dashing in.
You can't really zone the flash because he can duck fb at a certain range and punish non mb but doesn't mean u can't zone at all you just have to be more careful RM stance will get beat by mb fearblast.
Flash has high damage but requires meter and trait Sinestro doesn't and he has a vortex so the potential for damage is high as well.
Once Flash starts respecting you b213 is a gud option to keep him at bay and get some plus frames.
Charge trait then cancel and block low to bait out charge and punish with d2.
You said once Sinestro gets trait it becomes his favor well Sinestro can get 1 or 2 traits per game against flash since he doesn't really have many ways to stop him from charging trait full screen.
Once Sinestro has trait he can zone more and wake-up with Arachnid trait shot to keep himself safe limiting Flash's oki. His damage also increases.
There is more but this is all I can think of for now. Basically the match can go either way most important things is to build trait because a lifelead against flash can be gone in a second.
There was some great footage of a set of Cowboy vs. Honeybee but I can't seem to find it.
And here is some footage of me playing against SNKE
BLTB #31 Part 1:
starts around 1:25:00
He beats me all the time until recently I feel I have gotten a better grasp of the match-up
thanks
