[RichardJ]

Manifold vacuum is highest at idle and cruise. This puts negative pressure in the crankcase. I.E. air IN.
At WOT, manifold vacuum is very low or near non-existent. At the same, WOT produces maximum crankcase blow-by pressure. I.E. air out.

While this is going on, the turbo is dumping a lot of oil into the same rocker box and they're both trying to get out.

Valve cover breather or Tube to the air cleaner, this is always a two direction path for a PCV System, depending on engine load, turbo or not