Often you will want to develop a new screen or make changes to an existing screen. It's helpful to try each change out as you progress through your design process.
Test Mode is a great way to do this. You can make changes in the relevant screen designer and then test these out on your device. Make more changes and test again. This without having to publish a new version of the screen every time.
It's a simple and easy way to iterate on your design, figuring out formulae and other functionality you desire. All you need to do is to hit the TEST button in the designer, then choose who will participate in the test.
Note: The Test button is only available on screens that are in Draft status. If you have a Published version, then you will need to first hit the New Version button. A new version of your screen will be available in Draft status.
When you put a screen into Test mode, you must nominate the particular users that should see the Test version. Often this might just be yourself as the designer.
While a screen is in Test mode, the Published version (if one exists) will remain visible to everyone else. So you are free to change the screen design as much as you like, without fear of disturbing the stable production version of your screen. As long as the form is in Test mode, your testers' devices will update every time you save changes to the form design.
This makes it easy to make a change, save the design, then jump over to your device to test it out.
The only thing you need to ensure is that an app synchronization occurs. So that the changed design is downloaded to the device. You can do this by simply sending the app to the background and bringing it back to the foreground. Otherwise, you can click on the Check for Updates button in the settings screen off the app.