Answer first
Keratin is the main protein in hair. Hydrolyzed keratin is broken into smaller pieces, including 18 types of amino acids, so it can sit on and condition the hair surface. In the Dr. M Protein Tonic this helps hair feel stronger and less prone to breakage when brushing. It is a cosmetic effect, not new growth.
Hydrolysis
Breaking protein into smaller fragments makes it lighter and easier to spread in a tonic.
What you notice
Smoother feel, fewer snapped hairs in the brush.
Questions
Is it leave-on?
Yes.
Will it weigh hair down?
It is light; use the directed amount.