When you insert a finger into your anus and then tighten and relax the muscles around your anus you can feel there are two surrounding muscles, as you can also see on the drawing. To prevent any discomfort when something enters or leaves the anus it’s important to know how these muscles work.
The muscles are very close together and function independently. The external sphincter is controlled by the central nervous system. You can make it tense or relaxed at will. The internal sphincter is controlled by the autonomous nervous system, like your blood pressure and heart rate. To know how to relax this muscle will take time.
To learn to relax the internal muscle you have to get feedback from your body. The way to do this is to insert your finger in your anus and discover when it is tense and when it is relaxed. By doing this regularly you’ll discover how your body functions. You can learn to relax the internal muscle at will, but only by paying attention to the information your finger(s) provide. Deep breathing often helps to relax.
The anal tissue and the muscles are very flexible. When someone is really relaxed the anus can be dilated to such an extend that an entire hand can be inserted (when done during sex this is called “fisting” or “handballing.”)