The acuteness of the arrowheads and their opposing directions make it difficult to judge which piece of horizontal line is longer, the left or the right. When the acuteness is zero, there is no illusion and setting the midpoint to 0.5 produces lines of the same apparent relative length.
The sharper the angle, the more the left line segment appears longer than the right line segment. After adjusting the midpoint slider so that the lines appear of equal length, comparing the value of the midpoint to the correct value of 0.5 shows the magnitude of the perceptual error. Experiment with different degrees of acuteness to see how it influences the magnitude of the illusion. Remember to close the animation controls after reading the value, because it is not much of a challenge if you can see the value!
Use the top slider to adjust the acuteness of the angle of the arrowheads. Move the bottom slider so that the two line segments appear to be of equal length. Click the + button to the right of the bottom slider to read the midpoint value. If the lines are the same length, this value will be 0.5.