Let me give you a visual. It might help.
You have two couplers, a male and a female. As implied by the gender, the male coupler is inserted into the female. The male has a rib along the shaft. The female coupler has teeth on the inside. When you insert the male into the female, the female locks onto the male by biting onto the male's shaft with her teeth.
The tool goes around the male shaft. Start at the base of the shaft and slide it towards the female. The tool will enter the female to push her teeth of the male's shaft. If you do this very carefully, you'll be able to feel the teeth release from the male. It doesn't take a lot of pressure to get her to release her deathly grip. Make sure you are not pulling the male and female apart at this time. It will only make her teeth grab tighter. Once you got the teeth to release, you should be able to pull the male coupler away from the now unlocked female. Keep in mind that the tool stays with the male. You must hold the male coupler so you can pull it away from the female while pushing the tool towards the female to keep her teeth unlocked. It is an awkward motion. Try holding the male with your last two fingers and your palm. Use your first two fingers and thumb of the same hand to push the tool towards the female. Hold the female and only the female with your other hand and just keep her steady. Don't grab the tool with the hand holding the female. The tool stays with the male.
Try that...