I got the same expaination from Hans, well almost. Thanks 44Dwarf, that makes it a lot clearer.
Hans said, "Barry, the breather has several functions. First one is to keep the
the crankcase low. Otherwise if both pistons are down, you would have
say 2 atmosphere
pressure in the case. This could possibly break the cases and might
blow oil thru
seals. What you need for this, is a breather that opens momentarily
when both pistons
are close to the lowest position.
If you are at idle, the situation is close to this.
When you open the throttle, some gas will start leaking past the
pistons. This is
sometimes called blow-by. This gas has to get out thru the breather.
This now requires
a sizeable opening but still much smaller that the HD size, and that
opening has to
remain open long enough. Timing is still basically as above, but you
timing a few degrees since gasses always lag a bit with respect to
movements." (Per Hans.)
Thank you all!