This restricts the rectangle so its height/width ratio is a whole number.
This creates a rectangle symmetric around the starting point of the mouse drag.
Initially, the two resize (square) handles are in the top-left and bottom-right corners. Resize handles change the width and height of the rectangle in its own coordinate system, before any transforms are applied.
drag | Drag a circular handle to round corners |
Initially, the two rounding handles are in the top-right corner of the rectangle.
When rounding corners, dragging one rounding handle keeps the corner circular if the other remains at the corner. You can drag each of the handles independently for an elliptic rounded corner, or drag/click one handle with 
to make it circular again.