This variable limits the valid range of the time_step variable to a minimum frame rate. Below that frame rate, time_step stays constant, limiting the maximum per-frame distance of movements.
Consequence: The game will run at different speed on slow pc´s.
If time_step stays constant, the speed of moving objects will be determined by the framerate.
The framerate is different on every pc.
This will occur only on slow pc´s as said. When the framerate goes under fps_min (in you example 55 fps), time_step will be constant.
