Greetings guy,
From my understanding, the anus like the toung
at the roof of the mouth helps open and close the
curcuit, but it's used in counjunction with reverse breathing, i'm not aware of it being used with natural breathing.

As for when to do it?
Only after natural breathing is perfected.

The correct method of reverse breathing is,
slightly contract the diaphram while lifting the anus. on the exhale release the anus and expand
the diaphram.

As the count said, i'm unaware of this type of method being used by external styles. so be careful, incorrect practice can bring bad side effects which differ from person to person.

Hope this helps

Peace
Maoshan