Bryce,
It's honestly hard for me to explain scientifically.. The best way I can explain it is your body being forced to adapt to insane intensity that is ever changing. CrossFit is circuit training on steroids. The weight loads are not always crazy heavy but the amount of volume you are doing in the amount of time is the sick part. You don't rest either.
One of my favorite Cross Fit workouts is called "Fran".. Its a superset that combines the Thruster (Front squat & shoulder press) with Pullups. The workout is done as prescribed with 95lb barbell for your Thrusters, and bodyweight pullups. It goes 21 reps of each, 15 of each, and 9 of each. This is done back to back, etc.. with no rest. The workout itself takes about 5-8 minutes but the smoke factor is insane..
I have been lifting weights seriously since 2003, and over time, its just hard to keep it fresh, but CrossFit allows me to do that. I still bodybuild, but its more like do 1 or 2 crossfit workouts, then finish off body with light weights. When I returned from Afghanistan I was up to about 220lbs and just was not in good shape (lack of cardio and crappy food).. but now 7 weeks later I have shed 30lbs and am getting back to my normal weight and definition..
Check it out ..
www.crossfit.com or search crossfit on youtube..
Don't knock it til you try it