I think what Auron is referring to is how back pressure in the exhaust system can actually improve low end torque/power. It is my understanding that this is typically the case when it comes to high lift & especially long duration cams where with enough overlap between the exhaust and intake strokes that what can happen is as fresh air & fuel is drawn into the cylinder some of this air & fuel also escapes out the exhaust valves resulting in overall less power.
A solution would be to use lower lift and/or shorter duration cams and try to eliminate any valve overlap (the time both the intake and exhaust valves are open at the same time) however this limits top end power. Take the Concourse for example, same engine as the zx14 but its "tuned" for more low/mid range power, most likely this is done by tuning the cams, exhaust system and intake tracts.
The other solution would be to use back pressure that is always present in the exhaust system to help keep some of this fresh air/fuel in the cylinder at lower RPM. As Hub mentioned, there's a lot of science involved with this and its basically "black magic" to anyone who doesn't understand the science. But in general what I have read is basically whenever you have a wave (exhaust pulse) which hits an area of different pressure (outside air where the exhaust system ends) not only is this gas sent out the pipe but it creates a reflected wave that goes back to the engine.
Its this wave that causes the back pressure, engineers can and do calculate out the resonance frequency for this pulse and can tune it to improve performance in the lower RPM range for a very narrow range of RPMs (think 2,500-2,750RPM). Its my understanding that there is a similar phenomenon on the intake tract which is why the velocity stacks (the tubes from the airbox to the intake valves) are a specific shape and length. This could often be seen on a dyno graph as a sudden spike in power in the lower RPM range.
So... what does this really all mean at the end of the day? Like Hub already said, guess and check. Put your new exhaust system on, run it on a dyno (if you can), look at the torque curve and adjust the fuel as well as the timing maps (if possible) to get the best power possible.
As a side note, many cars now have variable cam/valves to remediate this issue, short duration/lift for low RPM power, long duration/lift for high end performance. Also there was a motorcycle (Honda? Yamaha?) that had variable length intake tracts in order to squeeze the most performance out of the engine.