From the radius of the unit circle of 1, it can be seen that when AB rotates for half a cycle, the point P just moves from A to B (as shown in Figure 1 to Figure 3), AB continues to rotate, and the point P moves from B to A, so it can be seen that the point P moves in a cycle with AB rotating for half a cycle.

In a period, two points from P to O are randomly selected (as shown in Figures 4 and 5). It is known that the trajectory of point P in Figure 2 is always on the right side of AB, so A is selected.

In addition, because the angular velocity of AB rotation is constant, the distance from point P to the center of the circle is gradually shortened from A to O. If the trajectory of point P from A to O is divided into several equal parts according to the rotation angle of AB, each trajectory will be gradually shortened, that is, the shape with the outer tip and the inner flat in Figure A will be formed.