The breathing must not be forced, but enough air must be furnished to produce the proper full vibrations.
BREATH MASTERY What then does perfect control of the breath mean?
1.

Ability to fill the lungs to their capacity either quickly or slowly.
2.

Ability to breathe out as quickly or slowly as the occasion demands.
3.

Ability to suspend inspiration, with the throat open, whether the lungs are full or not, and to resume the process at will without having lost any of the already inspired air.
4.

Ability to exhale under the same restrictions.
The above four points are common to speaking and singing, but singing involves further: 5.