DOWN AND IN - UP AND OUT
To properly or fully benefit from Tai Chi it is important that we understand the breathing technique involved - and coordinate our movements with these breaths.
Tai Chi functions (when done much faster) as a martial art fighting form - and in this respect - individual postures may be identified as either 'offensive' or 'defensive'. Accordingly, all defensive postures (or 'moves') are performed on an in breath - and all offensive postures/moves are made on an out breath. Furthermore, a defensive move is classified as a "Yin" and an offensive as "Yang". So the theory is: Defence on the Yin or In breath and offence on the Out or Yang breath.

THE YIN YANG SYMBOL
On the grand scale, the interlocking black and white shapes within the circumference of the circle are representative of the cyclical nature of the natural world. Day becomes night, summer becomes autumn, hot becomes cold … and life becomes death. The day is represented by the white section and night by the black. The dot (or 'seed') of black in the white and the seed of white in the black implies that 'nothing is all black and nothing is all white' … and therefore it follows that there is nothing that is 'all good' and there is nothing that is 'all bad', 'all right' or 'all wrong' … … … and so on. Essentially - the black section of the Yin Yang represents the YIN - or as I call it, the Down and In. So accordingly, the black/Yin represents the passive, the slow, the quite and withdrawn. The thinnest part of the black is 'minimum Yin' and the thickest maximum Yin. Maximum Yin moves on to 'minimum Yang' i.e. the thinnest white bit.
The white YANG section represents the aggressive, the quick, the loud and bright - with the thinnest being just 'dim' or 'luke-warm' and the thickest being dazzling or 'white-hot'. Thus, the 'not very loud' and 'not too bright' or 'minimum but more than nothing' Yang more or less overlaps (or is the same thing as) the maximum 'just a little muted or slightly dulled' Yin.

CIRCULAR BREATHING

Whether sitting or standing - we imagine/visualise that we begin each breath cycle by picturing that we begin breathing in at the base of our spine (where our tail would be if we had one). As we breathe in we picture the breath coming up our backs - and we reach 'full' as the breath reaches our neck. We then 'hold full' as the breath goes over our heads - and begin to breath out just as we picture the breath cycle reaching a point between our eyes. We then breath out, picturing the breath reaching our belly. We then 'hold full' as we picture the breath slipping underneath us - and begin to breath in again just as the cycle reaches our tail again.