I think TVS’ explanation is pretty close to the way it works, but it’s kind of tough to judge by the wording.
Here’s the basic dilemma. (Most of these examples are going to be from CFE because i had access to frame data in that game, but i don’t see any reason for it to be different from CvS2 or even from older Capcom games.)
If you charge down with Chun Li, then do Lightning Legs, you lose the SBK charge. There’s a hard-coded rule that the same charge can not be used for two moves. In fact, doing any special move resets charge. There are exceptions, as with SSF2T Balrog but those were generally specific to that game.
The problem is in the way people think of minimizing charge time. Most people seem to think that if you want Guile to do two Sonic Booms as close as possible to one another, you should charge DB, then tap F, then quickly go back to DB and hold it for as long as possible before hitting the punch button. The idea is that you get to “recharge” for the next Sonic Boom before you even do the first one. But that’s clearly not possible because charge is reset as soon as you hit the punch button. It’s good to be holding DB when you do hit the punch button, so that you start charging IMMEDIATELY after, but that’s the best you can get.
Now, there’s this other principle in play. Every attack causes a specific amount of hit freeze. In CFE, most light attacks cause 9 frame hit freeze, medium attacks cause 11 frame hit freeze, and hard attacks cause 13 frame hit freeze. So in that kysg Guile vs Hydron clip, he charges DB, then hits Hydron’s limb with c.MP and as soon as the punch connects, he taps F+LP to do the Sonic Boom and then immediately goes back to DB. The Sonic Boom is considered to have been executed as soon as he pushes the punch button, but it doesn’t come out until after the 9 frame c.MP hit freeze. So if he’s precise enough, he can shorten the charge period between the Sonic Boom and the Flash Kick to maybe 53 frames instead of 60 frames.
This really comes into play in a LOT of combos, because of course the type of hits landed inbetween charge moves affect charge time. Another thing it means is that doing hard attacks in the middle of combos actually gives you more charge time than doing light attacks, because there’s more charge time due to more hit freeze.
Anyway, i just wanted to get all this out there in case anyone cares enough to get all the facts straight about how charge works.