I know i'm quite late, but anyway....
The ideal conditions for learning the speed loop - it's quite good if you're a little owerpovered, sail about 5.5 m2, 100litre board.
Here's what you do
you turn downwind, move your back hand FAAAR back - as far as you can, and then you take off, then there are basicly 3 things you have to do:
1. pull on your back hand REALLY HARD
2. tuck in your back leg
3. look back
you really need to do that all at once and as soon as you do it you will start to rotate. Then you just hold on. At first you will land in waterstart position and you will hit the water with your back. the most important thing: DON'T THINK, JUST GO FOR IT!!!!
The first time I tried it (I'm 16 yrs old) I was on glassy flat water; just popped the board out probably like 30 cm and it worked really well - i landed safely in the water under my sail with my feet still in the straps.
The chances of getting hurt are as minimal as they can be, one thing to watch out for is that you're sure you've left the water (took off) before you do the 3 things listed above. If you dont, then i hope your footstraps are big enough because your sail will rotate anyway...
Good luck!
Dennis