The concept to siporax, matrix and marine pure is that they have a larger surface area for bacteria to colonize than live rock. Siporax apparently the most with something like 27 times more surface area, matrix with slightly less (20 times) and marine pure I believe even less but all three much more than live rock.
They do not provide food for bacteria but rather a place for aerobic and anerobic bacteria to colonize. The amount of bacteria is simply dependent on the nutrient load in your tank. So yes, they are a little like bio balls. Detritus will clog up the pores so ideally you want them in an area that has minimal detritus accumulation. And you can rinse them off, if you think it is necessary. They seem to do best in a slow flow area and as has been previously mentioned, positioned after pre filter(sock). Stacking them has been thought to promote less detritus accumulation and ease of rinsing if necessary.
They take probably 2-3 months to kick in (colonize adequate bacteria). They work primarily in reducing nitrates but somewhat on phosphates. The porous nature of the material provides colonization to both nitrifying and denitrifying bacteria.
Siporax is sintered glass and does not release anything in the tank. Marine pure is a different material (ceramic) and has been suggested as possibly a source of aluminum