Magnet tool
Use this tool to add magnets to a shape or to edit a shape's existing magnets.
Magnets are special spots on a shape that attract connection lines. If there are no magnets on a shape, all lines connected to it are attracted to the shape's center. If there is at least one magnet, each connected line is attracted to the nearest one.
Even if you haven't turned on Show Magnets in the View menu, an object's magnets become visible when you put the pointer over it with the magnet tool. Click and drag an existing magnet to move it, click a spot with no magnet to create one, or Option-click a magnet to delete it.
Generally, the magnet position presets in the Properties: Connections inspector should be good enough, but sometimes you might want to put magnets in very specific places on a shape. That's when the Magnet tool comes in handy.