Brass protection prevents distortion of brass and horn instruments that can be introduced by the declicking process, principally at higher sensitivity settings. This problem (which is not unique to VinylStudio) can, in severe cases, be quite noticeable as the following example shows. Notice the grey areas in the waveform display, which indicate a large number of click repairs in the regions affected (it's not normally this bad - we scanned this sample at maximum sensitivity with brass protection turned off to highlight the effect):
The result sounds like this (the difference is clearer on headphones):
If we zoom in, we can see what is going on (just to remind you, the green trace is the corrected waveform and the grey trace is the original):
As you can see, the 'spikey' nature of the waveform has fooled the click detector which has therefore made undesirable changes to the music - it doesn't look like much, but then it doesn't take much to be audible. VinylStudio's unique brass protection feature is designed to combat this problem by reducing the sensitivity of the click detector in passages of music such as this. Here is the same file scanned with brass protection enabled at the default setting (4):
And in closeup, you can see that the original waveform has been restored (or, rather, left unchanged):
Brass protection becomes somewhat less effective as the sensitivity is increased, so if a section of your music does sound distorted, or if there are a lot of solid grey areas on the waveform display, try scanning the affected section at reduced sensitivity (although, it should be noted, some forms of distortion present in the original recording can actually be eliminated by scanning at higher sensitivity, so don't be afraid to experiment).
Brass Protection can sometimes cause clicks to be missed. You can check this using the Corrections List, and if you think this is happening to you then scan again with a lower setting or even turn it off completely.