This sounds barbaric, and it kind of is. It’s literally removing dead flowers. I discuss this more in my harvesting blog post, but flowers each have their own unique stage of wanting to be picked to have the best vase life. Some will open more in the vase; therefore, they need to be picked right when starting to open, and some will open more once picked, so you need to wait, or they will wilt. Essentially, deadheading removes the blooms you didn’t pick in time, so the plant quits putting energy into them and can focus on making new flowers.
Once a flower is fully opened, it is trying to attract bees so they can pollinate it and make more flowers or reproduce. The flower has done its job. If you want the plant to make more flowers, don’t let its flowers go to seed. Deadheading spent blooms will keep the flower producing more flowers. Cut the stem of the spent flower just like you would if pinching. It will encourage more healthy stems to grow and create more flowers.