Sorry to disappoint those who thought this might be a post about sinks. I don't think I can handle recent plumbing trauma at this point in time and space.
So I'm standing in the basement classroom, trying to work out what the teacher meant when he said we had to sink. Sink where? Into what?
To sink is the other side of the coin from relaxation. These two qualities go hand in hand. When we can relax, there is a corresponding release in the body. The joints loosen and the body can settle downwards. This is the first step. Further stages in sinking and relaxation take place over time.
An important Aikido principle is to keep the weight underside. This is a product of relaxation and sinking. C.M Shifflett provides the following visualisation in his book, Ki in Aikido ( ISBN 978-0-9778702-1-9 Round Earth Publishing).
Imagine a shaft of light, like a fireman's pole going through the centre of your body, from the head deep down into the ground. Then imagine yourself gently sliding down the shaft or taking the elevator down as deeply as you can.
How does it make you feel? Play with it. Investigate it at different times.
Imagery is a key part of Tai Chi and is a powerful aid to posture, performance and more.