The dantian must have the continuous feeling of hollowing emptiness and accumulating fullness in order for the energy to fill the inside of your body, like air fills a balloon. Our whole body must do the same thing, like the balloon. Our arms feel longer, our back opens more, and our head reaches higher while our feet sink deeper. In order to open the joints and relax, we cannot expand in only one direction. It is necessary to expand in all directions, right must have left, up must have down, and forward must also be going backward.
Most people understand the concept that balance is maintained only with counter balance.
Still, T'ai Chi practitioners must not limit themselves to these six directions alone. Expansion includes every opposite diagonal in between. Expansion is the action of moving out in all opposing directions simultaneously. Expansion creates openness and space. More space means more emptiness to fill.