These aren't called blockstrings, they're called frame traps.
A blockstring is something such as crouching LK, LP, LK. Blockstrings keep your opponent stuck in block so you can keep them where you want them and continue pressure.
Frametraps, if done correctly, are to hit your opponent no matter what button he presses so long as he doesn't do a super/invincible move or block/parry. For example, with Makoto if you land cLK xx Dash punch and follow up with her command throw the person will get grabbed by it if they do anything other than jump or super.
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