Feel the outflow of air
Place the back of your hand close to your mouth and exhale. Feel the outflow of air on your hand. Now add a tone while exhaling just as before. Listen to how weak and breathy the tone is. Now produce more powerful sonorous or Louder in volume tone and you will feel that less air is expelled against the back of your hand. So, in order to make sonorous, powerful higher volume tones you must hold the air back instead of letting it all out at once.
Attempting to produce powerful tones during a large outflow of air is referred to as forcing. this is very strenuous and usually extremely wearing on the voice.
Exercise with a candle
Early Italian singing teachers were conscious of the importance of support. they put a lit candle in front of a singer's mouth to check whether the support was correct. The test was not to let the flame flicker while the singer sang; that is how little air should be released. This exercise should always work with speaking for training before speeches.