Maybe this will help everyone understand cyl pressure in relation to rpm and duration/overlap
One by-product of larger overlap is to reduce cyl pressures at lower rpm. with a factory cam the valves are not held open at the same time for as long as they would be with a higher duration cam. As a result the pressure built up in the cyl on the compession stroke remains higher for the hole time the piston is traveling up the bore. engine RPM will have less affect on cyl pressure.-- However if a longer duration cam with more overlap is used in the same engine, the cyl pressure will be lower at lower RPMs what will happen is that both valves will both be open at the same time for longer periods allowing more of the pressure to bleed off making less power at lower RPMs. Only as the RPMs increase will the pressure increase and produce more power till other factors start to take affect (heads or exhaust cant flow any more,carbs are too small ect)
One way to understand this is to imagine placing your finger over the end of a bicycle hand pump, partialy cover the outlet, if you push the handle (piston) slowly, the air will escape past your finger (valve)and you will be able to push the handle all the way in without much effort. But if you push the handle real fast you will find the pump becomes very hard to push as the air can not escape fast enough hence the change in low end power being lost due to more overlap.