There is an inbuilt accelerometer that detects when the section is impacted by a marble. This in turn triggers the sequence of LEDs.

This track section is built around the TinySaber from TinyCircuits. With minimal coding knowledge you'll be able to adjusts the sensitivity of the accelerometer, as well as the brightness, colour and speed of the LEDs by changing parameters on the clearly documented code I provide. If you have the programming skills to go a bit deeper you'll be able to fully customise the strip any way you like.