The original meaning of hack among computer folk is just something like "a clever way to make something easier to do". This is the sense in which it is used in MindPerformanceHacks.
Think of your memory as a muscle you're training by going to the gym. By spending a little effort in exercising your memory with the mnemonics in the book, you'll be able to increase the power of your memory significantly.
Also, most mnemonics in the book rely on some kind of meta-mnemonic that makes the mnemonic itself easier to remember. For example, in the NumberShapeSystem, it's easy to remember that the number 2 is represented by a swan because 2 is shaped like a swan.
What the book says is that the source code for the hacks that have any is available here: http://examples.oreilly.com/mindperfhks/
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