The pulse also changes by just touching with the finger. If I touch an acupuncture point too hard with my finger, the pulse becomes hard and/or fast, and if I touch an unsuitable acupuncture point for your body, the pulse also changes the same way.
The same thing happens to the abdomen and other parts of the body. If I touch a suitable acupuncture point with right strength, the abdomen becomes soft and smooth. So I check the abdomen and the muscle of the neck as well in order to confirm my pulse reading is correct, especially when the changes of the pulse is subtle.
That the pulse becomes harder and faster means your body gets stressed, while that the pulse becomes softer and slower means your body gets relaxed, which is the condition that acupuncture tries to lead to.