An angled diamond wheel is a type of grinding wheel, an abrasive tool. It isn't terribly different from a more technologically advanced form of sandpaper. The short version of it is that a metal powder is sintered (melted) onto the surface of the wheel, with small bits of abrasive (in this case, diamond) distributed throughout. Porosity is a matter of the spacing between those abrasive bits; the wrong amount will diminish the wheel's ability to cut, either by becoming clogged with debris or by not efficiently removing it from the cut, akin to the way that sandpaper will accumulate sawdust over the used surface, which reduces its friction by filling in the gaps (pores) with material.
Addressing changes in pressure and molding systems is a little too broad to approach without having more specific details to work with, but in general the hot press system uses lower pressures and therefore can work with lower-quality molds, although these molds are likely to be damaged in the process.