Colonic inertia is when the colon does not work properly to propell fecal matter through it due to a lack of motility....
Here are my favorite links that explain what Colonic Inertia is:
http://www.constipation-remedies-for-all.com/colonic-inertia.html
http://www.youtube.com/watch?v=CSjA4ceg0m0&feature=player_embedded
http://www.aboutgimotility.org/
http://www.med.unc.edu/