The answer is hydroxide relaxers. They're strong enough to swell the hair shaft to roughly double its normal diameter while processing.
Thio relaxers work through a different, gentler mechanism and don't cause that same level of swelling. Neutralizers and waving solutions aren't relaxers at all — neutralizers stop chemical action after processing, and waving solutions are for perms, not straightening. That extra swelling with hydroxide products is exactly why timing and strand testing matter so much: push it too far and you're looking at breakage.
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