Here is a extract of the FAQ- E-book, you really should read it.
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34.1. The 15s in HST
Before you give up completely on the 15 rep range, try lowering the weight... The 15s will accomplish what they are designed to even if the weight isn't that heavy.
It just has to burn like the "dickens". So, slow down the pace, squeeze and stretch, and try to enjoy them. You should feel tired afterwards, but there isn't any real benefit (size wise) from killing yourself.
The 15s are designed to condition your tendons for the upcoming heavy loads.
During the first 2 weeks of 15s, you are only going for intense burning. You want the kind of burn that starts small but then by the end of the set you can hardly bear it.
Do whatever it takes to both get this burn, as well as increase the weight every workout.</div>
and...
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Utilizing lactic acid as a stimulus for tendon repair/health
Now HST incorporates a few other things such as higher reps (for lactic acid) to prepare the muscles and tendons for future heavy loads. This serves as "regular maintenance".
Without it, you increase your risk of chronic injuries and pain. The metabolically-taxing reps enhance healing of strained tendons.</div>
That should give you a crystal clear picture, however this is not mandatory but it certainly helps, I use it each time.