source -> line -> component -> short line -> termination

the "lines" in this case are signal traces. works a lot like a SCSI bus. high speed switching operations can cause reflections on the line (in essence, this is resonant noise caused from high-speed switching on the lines as the memory controller gates on and off to place data on the bus, nothing more than rising and falling voltages). clearly enough, the faster the signal switches (higher frequency memory) the more noise.

termination resistors, used in passive termination environments, are exactly matched to the target frequency for operation based largely on trace length, ESR (equivalent series resistance) and the over capactive/inductive nature of the circuit, as well as some other things well out of the scope of this post. problem is that this is often matched for a specific frequency...which as we know is not so cool for overclockers. enter the active termination system. this actually uses a voltage bias to set final line resistance value so that the user can tune for better operation when overclocking.

this method of tuning is largely trial and error as you have no way of know the specification of the circuit or have any idea of the calculations need to find the "perfect" value. so anway, the answer is, strangely enough, whatever works best.

hope this helps.

FCG