The breakpoints menu is located in the bottom right corner of the main window. By default, no breakpoints are defined. From the menu you can choose "Edit Breakpoints…" to open the breakpoint editor window.
You can add breakpoints by clicking the + sign. By default the current size of the canvas is used when a new breakpoint is created. You can choose predefined breakpoints from the menu or create custom breakpoints. Breakpoints can be added, removed and edited at any time.
After adding breakpoints, new states are available for each widget. You can turn on the states in the style inspector to make specific settings for each breakpoint. A switch appears behind the states name on mouse over. The states can be activated by choosing breakpoints from the breakpoints menu or resizing the window.