I'll try to clarify the docs on the extra mode in the next release. In the meantime here's a quick summary:
There are three basic modes in WavPack: fast, normal, and high. The modes indicate the complexity of the decompression (or decoding). High is the most complex and is about half the speed of the normal mode, but compresses better in lossless and gives better quality in lossy. Fast is somewhat simpler than the normal mode and decompresses somewhat faster, but also is a little worse in compression and quality. WavPack is normally a symmetrical compressor, which means that basically the same process is used for both encoding and decoding, and so they take about the same time.
The extra option is exactly that: it does extra processing during encoding. It works in any of the three modes to improve them (in both compression and quality) and it can be very slow. However, it doesn't change the actual mode used so the resulting files will decode just as fast as they would without the extra option; it's free in a sense. I recommend that the extra option simply be used by itself, that is with no numeric parameter. The extra mode tends to improve the faster modes more than the higher modes (simply because there's more room for improvement in the faster modes).
There is an optional numeric parameter for the extra mode that (like I said above) I don't generally advise using. This is because it is not just a linear scale; instead it actually turns on different types of optimization. It is ordered so that it does more the higher the number, but that doesn't mean that the amount of time spent will give a corresponding improvement in performance. For example, in the normal mode, I just measured these:
normal: created john.wv in 1.92 secs (lossless, 39.32%)
-x1: created john.wv in 3.50 secs (lossless, 39.32%)
-x2: created john.wv in 5.81 secs (lossless, 39.32%)
-x3: created john.wv in 11.53 secs (lossless, 39.53%)
-x4: created john.wv in 25.98 secs (lossless, 40.28%)
-x5: created john.wv in 39.48 secs (lossless, 40.29%)
-x6: created john.wv in 82.66 secs (lossless, 40.33%)
You can see that even though the processing time just about doubles each increment of the extra value, virtually all of the improvement occurs at -x4. Before and after that you don't get much for the time spent, so when you select -x in the normal mode you get level 4 by default. Similar reasoning was used to choose the default extra level for the fast (-x6) and high (-x3) modes.