Actually,that is not speedreading but skimming!
Speedreading is a technique used,by controlling the eyemovement.
You control the eyemovement to move fluently over a line of text, in a slightly faster than you can read way.
You then recall the line of text you just saw (your brain is processing that line), and you will understand the line after when you already started reading the second line.
With speedreading, contrary to skimming, you do read every word the author has written. Often you can accelerate your reading speed by 4 times or even more.
Speedreading has the theory behind that the brain can decode text faster than people can talk. For those of us who read regularly it will start to come automatically.
With speedreading you tend to always push your limit, and reduce the amount of re-reads (the need to read a line back when not understood).
The book about speedreading is an expensive book, I once bought, and backs up a lot of research data.
It seems the brain is upto 80% more active reading this way than reading regularly.
It also talks about handling stress you get when reading in such high speed ways.
I can say all the book has to say here on the forum, but I'd rather suggest you to buy and read the book yourself (don't have it here,& forgot the name).
Wikipedia defines:
http://en.wikipedia.org/wiki/Speed_reading
and:
http://en.wikipedia.org/wiki/Skimming_(reading)
Skimming is not as intensive to the brain as speedreading; and while you do get the point of the writer,you can't really apply that for science books, or texts where 'every letter' / sentence counts.