In the first window, use the mouse to sketch a signal x(t), or select one of the provided signals. In the second window, the independent variable (on the horizontal axis) is v. Clicking on the v axis will set a value of t, and x(t-v) will be displayed as a function of v.
If you click to set t at zero, the flipped signal x(-v) is shown. The value of t can be dragged with the mouse to show how the flipped signal is shifted for different values of t.