Protein-derived substances like hydrolysed wheat protein, leucine, or arginine HCL are used to temporarily protect and repair each hair strand. They are attracted to keratin (the protein in the hair) covering the strand and sticking to it so the split ends come back together again.
Hydrolysed wheat protein is a powder derived from wheat through a chemical process called hydrolysis, which involves the reaction of the substance (in this case wheat protein) with water and an acid. The process is necessary to make the wheat protein small enough to access keratin through cuticles breakage.
The protein will increase the tensile strength of the hair to make it less susceptible to breakage. It also adds volume, texture and condition to the hair, preventing it from drying out and leaving it soft and silky.