YoungJohn,
The general theory is that when looking at a single cylinder it is essentially and air compressor. Normally, we think of the engine as comressing the air in the combustion chamber at top dead center when the exhaust valve is closed. But, if you think about the underside of the piston, when it travels back down to bottom dead center, it is compressing the air inside the crank case because the volume inside the crankase is getting smaller. With the D breather, which is simply open to the atmosphere, air is pushed out as the piston descends and is pulled in when it ascends. Thus, it lets air in and out of the crank case so that with every revolution, the crank case experiences an amount of vacuum and an amount of pressure. If you put a check valve or PCV valve on, the idea is that the valve does not allow air back in so the crancase pressure should be lower through most of the cycle. The timed breather does the same thing by opening a slot when the air is going out and closing the slot when the piston would normally want to draw air into the crank case. You can now understand that having an atmospheric breather on the bike at the same time as you have a timed breather is not productive. When the timed breather closes as the piston goes up, instead of drawing a little bit of a vacuum, the outside air simply comes in through the D breather. It is best as Timetraveller says to go with one or the other, not both. You can put a check valve in the D breather line in the hopes of holding some of the low pressure in the case.
I have used the timed breather routinely to 7000 rpm with good results, but I have also used a hose with a Ducati pcv valve with the same results. I cannot remember if the D breather needed the modified top guide to work properly, but I hope someone who does know will jump in.
David