Technically it’s useless to do a throw / Dragon Punch option select if the opponent does a reversal throw. It’s a myth.
In this example Guile attacks with a crossup Short and is fully charged for a Flash Kick once he lands.
The defender is in blockstun for 11 frames and is throwable on the 12th frame.
Assuming Guile does an OS back Fierce kara-cancelled into Flash Kick.
On the 12th frame, the startup of Guile’s back Fierce begins, but will not be kara-cancelled into the Flash Kick until the 3rd/4th/5th frame.
Which means if Guile attempts a throw on the 12th frame, he will not be invincible until the 15th/16th frame when the Flash Kick activates.
Guile can throw the opponent, but he can also be thrown by the opponent.
If the opponent does a perfect reversal throw on the 12th frame, the same time as Guile, then the game will toss a coin and give it 50% of the time to either Guile or the opponent.
Now let’s say the opponent does not do a perfect reversal throw and times it a little late, say on the 13th or 14th frame after blockstun. Then Guile will throw him, and the Flash Kick install will be useless.
Let’s say Guile does his throw/Flash Kick a little bit late but the opponent does a perfect reversal throw on the 12th frame. Then Guile will be thrown, also making the Flash Kick install useless.
No matter which way you look at it, installing a Flash Kick or Dragon Punch after a throw attempt is useless if the opponent does a reversal throw. You will either throw him or get thrown depending on the timing.
The Flash Kick install after a throw only works if the the opponent does something else besides a reversal throw, such as an airborne/invincible attack like a Tatsu or a Shoryu. Then the throw attempt will be kara-cancelled and the Flash Kick will kick in.
Other than that, this OS is another one of those ST myths that needs to be debunked.
All the Ryu player in that video had to do was reversal throw.