From my, admittedly very limited, experience as a skater, I agree with mskater93. You need some \"prerotation\" to initiate the jump, although I do not consider it \"prerotation\", because this is simply how the jump looks like.
You don\'t see skaters approaching the loop jump on one leg, then springing up in the air and mysteriously getting the rotational power from nowhere. Usually they do either a \"two-foot\" approach, or enter the jump from 3-turns.
This \"need for momentum\" is what makes the loop combos so different from toe combos. In toe combos you can mess up the first jump - pop it or have an unsteady landing - and still tack on a double (in my case) or a triple (in case of the skaters better than me

) toe as long as you have some speed left. Even if you look at the combos of some top skaters you will notice that very often they have a visible slightly wobbly \"steadying-the-edge\" moment. For me it\'s a bit like doing two separate jumps.
The loop combos, on the other hand, use the momentum from the first jump - it serves the same purpose as the second foot assistance or a 3-turn entry when doing a single. The first jump needs to be executed perfectly, you cannot steady the edge and then launch off. A tiny moment of hesitation or a slight wobble and you end up popping the jump, or launching in the air under a wonky angle. I guess this is why we see the loop combos so rarely done by men - they certainly have the power to rotate the jump, but rarely great enough edge control on landing (and their grater body mass makes it more difficult) to be sure that nothing will go wrong with the second (or third) jump.
I suggest finding some videos of skaters that are famous for their good jumps and watching their loops - both single and in combo - frame-by-frame.
Most of the jumps I find rather difficult to prerotate - staying on the ice longer than necessary would result for me in losing the momentum rather than making the jump easier! One exception is the toe-loop, which many skaters toe-axel (and some are penalized for it by having the jump downgraded; or most, actually, since the rules baceme stricter this season). The name \"toe-axel\" itself explains what happens during the jump - instead of using the toe-pick merely to launch the take-off, the skater shifts the weight onto the assist leg, swings in and does sort of, well, toe axel.
Also I would like to thank mskater93 for the links to the biomechanics of skating articles - I am reading them now. Great stuff! Not only informative, but also presented in a very accesible way and amusing(I loved the flash animation with the skater being blown away by the wind

).