Hi John,
Most people find reverse breathing different than what they are used to doing. It is also very common for people to require a bit of time to realize it comfortably, or feel like they are doing it correctly.
You don't 'have' to use reverse breathing. It is recommended. I do it all the time I am conscious of breath/breathing. When it is uncomfortable or odd to someone to the point it hinders their relaxation, it is sometimes good to just let go of the focus on breathing and focus on relaxing and emptiness. Whatever gives you a sense of both working to improve suggested alignments/method, or employing them, but also not to the degree you that you limit your ability to relax. I would suggest some of each, if reverse breathing is not comfortable to you yet.
Perhaps focus some attention on it in your session, then let go and focus on relaxing, or the smile, or the visualizations given relative to the exercise/meditation.
Reverse breathing comes from Taoist method. Taoists were into longevity, and reverse breathing promotes that. That is the simplest way to express the primary benefit without getting off into lots of specifics that aren't necessary to know, to receive the benefits of reverse breathing. Any specifics can be gradually learned if you do some side study, but it is better in general not to base your practice in the academic specifics of qigong method. Immerse in the experience of your practice, and if further interested, then go to the books or online to learn more about reverse breathing, or five element theory, etc..
Trust Master Lin's lead in how you employ it in your practice. There is contradictory information out there regarding the extent to use it in your practice.
One tip I would suggest, is to focus on really filling up the lungs/upper lungs, when you inhale. Take your time and relax as you do it. Filling the lungs completely, helps counter any sense of contraction or restriction stemming how you hold in the abdomen on the inhale.
love,
gallen
