What part of the fin should not be used when donning fins?

When donning fins, it is important to avoid using the strap. The strap is designed to secure the fin to your foot rather than serve as a means of putting the fin on. Attempting to use the strap while putting on fins can lead to improper fitting or even damage to the fin. Instead, you should focus on inserting your foot into the foot pocket, which is specifically designed to comfortably hold your foot in place. The foot pocket allows for a secure fit and ensures that the fin operates effectively in the water. Thus, using the strap during the donning process is not advisable for achieving the proper fit and comfort needed for diving.