You don't need a button to slow down the robot, the robot you're pushing will do that for you
See if you can figure out what this chart is telling you. Notice that torque is zero when speed is 100%.
This is important stuff, it takes a while to understand it, so please take some time to think about it and discuss it more. It's likely a real expert will jump in and point us to a relevant thread, or give a better explanation than I can (I'm a mechanical engineer....)