Tai Chi or Moving Qigong class exposes you to the movements, then you must practice those movements at home. There are two ways to look at this: either as another burden on your life's full plate or as a chance to take a break from the rat race and let all the weight of the world roll right off your shoulders.
The very first movement you learn on the very first day of class is a fantastic QiGong relaxation exercise that can help you begin to dump stress, if you do it in the right frame of mind. However, if you only do the Tai Chi or Qigong movement to prepare for the next Tai Chi class, then its just homework, and it won't be that relaxing.
But, if you breathe deeply (see below video) and let every muscle of your body relax, allowing the burdens of the week to roll off your shoulders each time you practice the movement, it'll feel great! Refer to the "Free Online Lessons" section to see how movements can be used as a relaxation therapy, even as you learn it, by including deep full breaths. Learning in this spirit can make all the movements become relaxation therapies even as you learn them. More on deep breaths in the "Curious About Tai Chi & Qigong," and "Beginners-Enthusiasts" sections.