1.Amino acids (small not very useful)
2.Proteins (large not very useful)
3. Hydrolysed/Hydrolyzed protein (good size, very useful)
The hydrolysed protein will be attracted to hair and conditioner tends todeposit better in regions of damage ( Journal of Cosmetic Science, pg 265-279, 2004). For natural hair, damage to the cuticle can come from heat, combing, washing, manipulation etc. This is likely to lead to cracked, chipped or missing cuticles. For those who colour treat their hair with permanent colour (not henna - read more on henna here) then the cuticle is likely to be damaged by the lifting process needed to get the dye into the hair shaft (Chemical and Engineering News, pg 52, 2000).
Hydrolysed protein can help patch up the cuticle. Colour treated hair does benefit the most from this treatment.
Now on to Avril's question - Aphogee 2 step treatment contains hydrolysed protein and it is quite high up on the list (second after water) which means it is probably an intense protein treatment. I don't know the exact reason why hair becomes hard during the process of treatment ( I don't work for them!). My theory is that the heat is needed to make the protein bind better to hair before the washing stage. The reason why the hair becomes so hard I would think is because of the protein binding to the hair and probably to some extent to itself (remember the hint - what happens if you apply heat to egg white? Meringue as Keisha said lol).
Why would the heat be needed? Well, most wash off products do exactly that - wash off. Therefore applying heat may make some sense to try and get 'stronger' bonds between the hydrolysed protein and the hair. The final step to moisturise the hair is necessary to restore pliability.
I have never used Aphogee but there are hundreds of reviews raving about its potency with effects from mending split ends, instant strength and stopping breakage. I can't really argue with a 5 star rating given by 113 people.