Tsubos

Points in Shiatsu are known as Tsubos, a Japanese word meaning 'vase'. This is because the impression on pressing on a Tsubo is, first of all, some resistance, which yields as the point is held, whereupon there is a feeling of 'opening out' under the thumb, before connecting with the 'bottom' of the point. This is illustrated in the diagram below; even if you do not experience these perceptions immediately, you will do so if you practise enough, and learn to locate points more precisely. What you are doing, in effect, when pressing and holding a Tsubo in this way, is connecting with the Ki underneath the point's surface, many Tsubos being 'gateways' into a Ki channel or meridian. The way you experience Ki in specific points is known as the Tsubo effect.
One of the main differences between acupressure or acupuncture and Shiatsu, and one of the great benefits of Shiatsu, is that you can actually treat any point on the body, and not just the ones that are on the classical channels. However, the classical points are more obvious to locate, more straightforward to treat, and easier to learn the effects of; so you will probably do best to focus on these at first. Later, you will find that you can apply the same principles in an almost infinite number of locations.