I think the idea here is that you cannot control a bowling ball when it is in your pocket. As long as there is desire, there is only so much "control" you have of the mind, just as there is only so much control you can have of a bowling ball in your pocket. Many people bowl as if they have a bowling ball in their pocket -- very little control. But what about the expert bowler? Not so different from any professional athlete -- they have trained over a long period of time to learn how to use the bowling ball to achieve their desired ends -- a strike. This is similar to people and their minds -- most have little to no control. But just like the bowler, many Buddhists will train and meditate over a long period of time to learn how to "control" their desires to achieve a desired "end" -- a calming of the "monkey mind." At some point, with the bowler and with the meditator, all of this training and practice leads to a different relationship and understanding of the ball and the mind. Athletes might call it getting in the "zone" -- they are no longer consciously trying to control every situation -- they are an integral part of the flow of the situation and their actions come naturally, not contrived. They just seem to automatically know what to do with the ball to make the strike, but they are not really even all that invested in the strike anymore, they are just in the moment. They have achieved a union with the ball and with the immediate situation that normally they do not have in the rest of their life. In the rest of their life, they are much like most of us -- very little control of the mind -- because they believe, like most of us, that we are somehow a separate entity from our mind. In Buddhism, this duality is an illusion because everything is believed to be interdependent. So as the bowler gets in the "zone" during a game, a meditator, too, after many years of practice and training may find themselves in the "zone" -- in the place of understanding that they are not an unchanging, concrete entity separate from their mind or from anything else. They have come to the same understanding about the mind as the bowler has come to about the ball. This understanding cannot be forced, it cannot be controlled, that is the paradox. At a certain point, the more one tries to control anything, the farther from the goal they are, because the goal is to shed the illusion that we can have control. When that illusion is gone, then, in that moment, the false separation disappears. The thinker then becomes the thought, the feeler is the feeling, and the bowler is the ball.

Edit: changed "ever-changing" to "unchanging"