smallhands, thanks for asking that.
First, here's the book I was referring to earlier: http://www.amazon.com/Reading-Thought-Units-ebook/dp/B005GVL... It's a new method, which makes it very easy to practice doing what reading improvement instructors have been recommending for over 100 years -- that is, reading groups of words at a time.

The short answer is, visualizing ideas is actually 'thinking' more about what you read. Meaningful word-groups form ideas which you can visualized at just one glance.

The longer answer is…
Visualizing ideas is forming a mental picture or concept. A picture of course is simply imagining a thing or an action. A concept would be more like imagining an attribute or a description.

For example, look at this line from Alice in Wonderland:
"The rabbit-hole went straight on like a tunnel for some way".

This could be considered as 4 ideas, or 'thought-units':

The rabbit-hole

went straight on

like a tunnel

for some way.

You would look at the phrase, "The rabbit-hole" and imagine a rabbit-hole. And this doesn't have to be a very exact or detailed picture, but just the act of picturing it forces you to use the visual part of your mind rather than the verbal part. This makes what you read more meaningful and therefore improves your comprehension. It's only by improving comprehension that you can ever read faster. I mean, if you aren't comprehending something, you aren't really reading it, are you? You're just sounding out words.

The next phrase, "went straight on" is an action, but also more of a concept. You would imagine the concept of something going straight.

This idea of concepts is a bit harder to explain. It's just thinking about what something really 'is'.
One exercise which helps understand 'conceptualizing' is to try to look at things, and understand what they are, but without naming them. This can be surprisingly tricky to do because we are so used to naming things. But try to look around you, and think about what things 'are'… without thinking of their names. What's left in your mind when you don't say words, is the 'concept'. It's getting this 'idea' or message across which is the goal and purpose of language. The sounds or text are just the messengers.

Anyway, likewise with the next two phrases. "like a tunnel" can be imagined as a tunnel or the concept of 'tunnelness'. And "for some way" is another concept. You imagine the concept of something being of a considerable distance.

The idea of reading in word-groups has been around since 1879, when it was discovered that this is how fast readers read. Instructors began teaching exercises for students to try to increase their 'eye span', but really what was needed was to 'understand' more words at a time, not just 'see' more words.

The key to understanding groups of words at a time, is to take 'meaningful' groups, which form whole ideas. And the key to reading groups of words, is to visualize the ideas. This is because you can't really 'say' multiple words at the same time, but you can 'see' one whole idea at a time.

If you think this sounds hard to do or strange, then think of how we read compound words as single ideas all the time. We think of cupcake as one idea, not as 'cup' and 'cake'.

That's all this is, reading a group of words as the one idea they represent together. Fast readers have always done this. In fact they often describe how they read as "reading in pictures".

Sorry for such a long answer, but I hope it's been helpful.