say using something like a hartman mask, and focusing on the moon, the cut out circles allow you to still see objects, by not as bright because the area of collection of light is significantly reduced. my view point only and I might have it wrong, but I am happy to be corrected.
Like photography, it means having a higher focal ratio.
So an f/5 scope is a 10" mirror and 1250mm focal length. If I stop it down, by say using an aperture mask with a 100mm hole, my f/5 scope becomes an f/12.5 scope- albeit with reduced resolution because it's now effectively a 100mm aperture scope.
I haven't actually done it myself, but if the hole avoids the spiders and just lets the light pass between them, it would make the scope a "clear apperture", ie. it would behave more like a refractor and the diffraction spikes would dissapear, however, everything would be a lot darker and a lot less resolution. In some circumstances of bad seeing, stopping down may actually improve the quality of the image (so they say). The theory being that the bad seeing is produce by "blobs" of air, if the apperture is smaller than these "blobs", the image improves. Again I have never actually experimented with the idea, but I have seen some articles on it.
I've made plenty over the years. On a reflector it means no diffraction spikes. On bright objects it works well. Also great on bright close doubles such as antares/Sirius, makes them easier to split. If you make a three holed one avoiding the spikes, one can experiment with ie: having all 3 open, 2 open 1 closed etc. My best views of jupiter on my 12.5" reflector comes from 1 hole closed in a 3 hole mask.
stopping down the aperture camn increase contrast by cutting the glare out on bright objects... its not for everyone of course, but the beauty of it is that it doesnt cost anything... just cut a hole of say 80-90mm diameter in a piece of cardboard and place it over the front of the scope. its that simple. your best bet is to just try it for yourself
Petra, here's a pic of the one I made for my 12" lightbridge. I have since painted it flat black to improve contrast, but I don't know if it makes any difference because I didn't use it before I painted it. It has four 80mm holes to effective halve my aperture as I will mainly use it for the moon. Looking at the moon with 12" was absolutly blinding, litterally.