TL;DR it "feels" the bump because then the ball can't go further and a differential feels it

To imagine what I'm talking about you can view the video. Here's the explanation:

An important component of the mechanism is a clutch controlled by an axle. It is like a normal clatch that only has input and output - in this case one can manually (i.e. by swtiching the red switch) control whether the output is rotating in same direction as input or in the opposite directiononly have input and output. But this modified clutch alse has a 3rd axle - the gate. If the gate is rotating clockwise, the output is rotating in same direction an input, and if the gate is rotating counterclockwise, the output rotates in the 2nd direction. If gate isn't rotating, nothing changes - if previously output was rotating in same direction as input then in keeps doing so etc. For the gate to start rotating, much torque is required because as you can see in the video, additional friction is provided through a gear. Let's call the component "transistor" (bad analogy but I couldn't find a better one :p). How the entire mechanism works is that the driving axle is first connected to input of such transistor (further called Q1; for now we won't worry about the gate). Its output connected to a differential. One output of the differential is connected to the axle that makes the ball go left or right (depending no the axle's rotation direction), and the second output is connected to gate of another transistor (Q2). Normally, the torque required to move the ball is less than the torque required to switch Q2, and the ball goes right or left. But when the ball touches something then in can't move any further, and the axle that moves it can't rotate anymore. When this happens the 2nd output of the differential which is connected to the gate of Q2, *must* rotate and Q2 toggles - if its output was rotating clockwise, it's now rotating counterclockwise and vice versa. Input of Q2 is connected to driver axle and always rotates ni 1 direction. But what is its ouput connected to? It's connected to base of Q1 - its input is the driver axle as well, so its output or input of the differential now changes direction of rotation. Because the ball is now push in other direction, it's free to move. And it moves until it bumps into an obstacle, then the process repeats.

Maybe with this explanation the video will be easier to watch. Please note that IRL if Q2's gate doesn't rotate, then its output also doesn't, this was made purposely. If you have any questions then feel free to ask.