The first post on glycerin was getting a little bit too long so I decided to split it up. Please see the first post here which details the moisturising effect of glycerin.
This post is about the science. The way glycerin works is not fully agreed upon. The two arguments placed forward are
1. Glycerin penetrates hair - 'Glycerin is known to penetrate the fiber and is unlikely to localize moisture in the vicinity of the surface. '(J Soc Cosmet Chem, pg 39-52, 1985)
2. Glycerin doesn't penetrate hair (to be completely scientific, they are talking about glycerol which is pure glycerin without any water. Dewetting the surface means the glycerol holds onto the water present on the surface) - 'Glycerol is known not to penetrate the hair and it probably quickly dewets the outer surface of the scales.'(Journal of Colloid and Interface Science,pg 329–335, 2004)
In the diagram below, I decided to show how the additional water comes in. In the event that you mix glycerin with water, the relevant images are the last two on the right.
In my opinion, glycerin is quite small and likes water. Hair easily lets water through and as we all know, coconut oil too. Therefore I think glycerin does penetrate hair to some extent.