In Japan we use the expression "no time to breathe" to indicate a very busy situation. This expression means there is no time for calm, ordinary breathing. What do you have to do to return to ordinary breathing? First, exhale deeply. This brings relaxation naturally. When both mind and body are tense, the only thing that consciously leads to relaxation is controlled breathing. Add simple body movements to controlled breathing and the effect doubles.
Habits are formed by repetition of an action, consciously or unconsciously. By practicing extension and relaxation exercises every day at the dojo, Aikidoists develop an ability to switch back and forth between extension and relaxation. Many Aikido students find that practice helps them relax when they want to.
Taiso, Breathing and Relaxation
I have been considering the comment made in class last week, "It's hard to force yourself to relax." A Google search for Aikido + Relaxation returns page after page of thoughtful posts and columns by instructors and students in many different schools. One repeating theme is that relaxation can be learned. Here is a concise quote from Aikido For Life by Gaku Homma.