Thanks for your comment and for playing.
Sorry about the limited instructions, I will add a tutorial in an upcoming update that describes all the content.
The shield is activated when you touch the die with your tool. You can then use the shield to fend off the balls. However, your tool will be turned into a shield, after which you can no longer activate the walls.
If you store your tool, you can activate the walls and take breaks between sprints. The walls then act as a protective shield and the balls can no longer hit you.