Thanks, that's better than your first pass, though I'm still not buying the story. My social engineering effort worked though.
I'll freely admit I find this confusing. And that's with having studied this shit in school and worked in the financial industry (in trading no less).
There are a number of methods of getting inside or around the order book, most of which involve breaking rules. But the regulators aren't even playing catch-up, they're so far behind.
HFT is very time sensitive, operating at or within the 250 microsecond window -- that's the time a light beam takes to travel 75 km. So one way of jumping in on the order is being, say, a few thousand km closer to the exchange than some other trader.
As to limit orders (and you really haven't explained how the order book avoids this), by beating others to the trade on both sides of the order, the HFT can get in and spin the equity, taking a cut, or stealing your trade (you look to buy at $100, seller is at $99.99, HFT buys at $99.99 but finds a buyer at $100.01).
The point is that by inserting themselves between other traders, by virtue of speed, HFTs skim a proft. It's small (and was enabled largely by decimalization), but can be made up for in volume.
The saving grace is that the HFTs are up against one another (at least until they start colluding), so they're weaving complexity traps against one another that wear down the advantage. Though there's the risk of more flash crashes and other disasters resulting from processes they barely understand themselves.