Use this tool to paint an ellipse. The currently selected brush is used for drawing the ellipse outline. Click and hold the left mouse button to indicate one corner of the ‘bounding rectangle’ of the ellipse, then move your mouse to the opposite corner. Krita will show a preview of the ellipse using a thin line. Release the button to draw the ellipse.
If you hold Shift while drawing, a circle will be drawn instead of an ellipse. Holding Ctrl will change the way the ellipse is constructed: instead of two corners, the initial mouse position indicates the ellipse center, and the final mouse position indicates one of the corners of the bounding rectangle. You can press Alt while still keeping the left mouse button down to move the ellipse to a different location.
You can change between the corner/corner and center/corner drawing methods as often as you want by pressing or releasing Ctrl, provided that you keep the left mouse button pressed. With Ctrl pressed, mouse movements will affect all four corners of the bounding rectangle (relative to the center), without Ctrl, the corner opposite to the one you are moving remains still.