From my recollection of the short course I did on speed reading the key to the technique was to realise that to understand a sentence in english one doesn't need to read every single word in a sentence. Also the eye can capture more than one word at a time so the key is to break a sentence into managable groups and make your eyes "jump" from group to group without neccessarily reading every word. (If I remember the optimal was 3 groups but it depended on the size of the text and the page - for newspapers you should try and read each line in one go because the lines are narrow)
Once this relatively simple technique was understood it was then a matter of practice to increase your level of comprehension and retention while using the technique.
It was also pointed out that most people will read "non technical" text much faster than "technical" text - ie you will read a novel much faster than you would read a maths text.
From memory I believe I was timed at reading "non technical" at 450 words per minute with 80% comprehension and "techincal" at 250-275 words per minute with 70% comprehension.
However, I have found I rarely speed read when reading for recreation as it kind of defeats the purpose and you don't get to enjoy the talent (or lack thereof) of the author as much if you rush through the text.
I also found it was most useful for "wordy" technical text eg history, law, HR, psychology etc but not as good for science based text eg maths, accounting, statistics etc. I also found that in areas like philosophy and "english literature" I would miss some of the important nuances within the text by only focusing on the "big" words.