There are two questions or doubts commonly associated with Mahasi method: 1. Observing abdomen, 2. Using labeling.
The following three quotes are the explanations for observing abdomen and using labeling.
1) "Satipatthana Vipassana" by Venerable Mahasi Sayadaw:
A simpler and easier form of the exercise for a beginner is this: With every breath there occurs in the abdomen a rising-falling movement. A beginner should start with the exercise of noting this movement. This rising-falling movement is easy to observe because it is coarse and therefore more suitable for the beginner. As in schools where simple lessons are easy to learn, so also is the practice of vipassana meditation. A beginner will find it easier to develop concentration and knowledge with a simple and easy exercise. ---
Outline of Basic Exercises
When contemplating rising and falling, the disciple should keep his mind on the abdomen. He will then come to know the upward movement or expansion of the abdomen on breathing in, and the downward movement or contraction on breathing out. A mental note should be made as "rising" for the upward movement and "falling" for the downward movement. If these movements are not clearly noticed by simply fixing the mind on them, one or both hands should be placed on the abdomen.
2) "The Four Foundations of Mindfulness: A Summary" by Venerable sayadaw U Silananda:
You know the four foundations of mindfulness, four kinds of setting up of mindfulness. There are four because there are four kinds of objects.
The first one is body. Sometimes body does not mean the whole physical body, but a group of some material properties. Breathing is also called the body. Different parts of the body are also called the body. By the word "body" we must understand anything that is associated with the body.
3) "In This Very Life" by Sayadaw U Pandita---"Mediation Instruction" in chapter one:
In this meditation it is very important to have both effort and precise aim, so that the mind meets the sensation directly and powerfully. One helpful aid to precision and accuracy is to make a soft mental note of the object of awareness, naming the sensation by saying the word gently and silently in the mind, like "rising, rising...falling, falling."
As explained above, the reason to use abdomen is because it is easier to observe and it is also a part of “body”, which is within the context of “The Four Foundations of Mindfulness”. And, the reason to use labeling is to have precise aim at the object observed in order to develop sharp mindfulness, which is needed to penetrate the object/phenomenon observed in order to see the true nature of all phenomena.
The labeling itself will naturally drop off, not intended by the practitioner, as the mindfulness gaining its strength.
However, if the yogi is using breathing, also known as “body” (quote 2), instead of abdomen as the primary object, he/she could ask the Sayadaw for advice during the interview.