There are two types of swipe actions you can perform:
Swipe x number of times - each swipe will swipe 75% of the screen.
Swipe to an element - Will continue to swipe up/down until element is found.
Swiping the screen is done the same way it is done on your mobile device: Choose "Swipe" from the menu and swipe on screen in the direct you want to swipe to. Just make sure you keep the curser inside the view.
An action is added to the test steps
Swipe to element
Choose "Swipe to element" from the menu. You can choose to swipe to a specified text, a resource ID, Class name or Accessibility ID. You can choose whether to tap on the element once found or not. 21 will swipe up to 30 times and try to find the element before it declares failure.