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:
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).
Actually, almost never. It rarely causes problems and frequently protects your music from damage, especially at higher sensitivity settings. The most likely problem you are liable to encounter is an unrepaired severe click in a passage of, say, saxophone music, and it is often quicker and easier to repair such a click manually. However, if you have a recording in very bad condition which consists largely or entirely of 'mellow tones', you might get better results with brass protection turned off.