Estragon has really bad foot pain from the boots and so this pin would help him alleviate it. This pin is also good because it gives a description of how to use this water bottle method, and Estragon is a little childish and has to have things explained to him so it is perfect for him.
Got knee pain? You probably need to work on your ankles. Over time, especially if you spend a lot of time sitting, your ankles lose range of motion—which forces your knees to compensate (and complain). Do this move as part of your warm-up, or anytime you like. You'll see the benefits in your walk, run, or strength workout almost immediately.