Substrate contacts

Hi friends,
basically substrate contacts are used to reduce substrate current says my text book. But i am not able to understand how it reduces it? Can somebody explain it?
Thanks in advance

It reduces parasitic structures, in particular parasitic NPN transistors that can cause latchup. These form over silicon isolated from the substrate by oxides.

Thanks for the reply
Can u explain little bit in detail?

If substrate current flows through the parasitic NPN structures generated in standard IC processes, it can cause latchup as duffy stated, so you want to shunt the current away from these structures. The substrate contacts provide a short path for these currents out of the substrate to minimize the occurrence of this latchup. I don't see how it would "reduce" substrate current, though.

^ Yes, I think they are stretching a point - the parasitic structures are in the substrate, so...

Thank u guys