The counter IC we used is called an "Up/Down Counter" because it can count the number upward from 0 to 1, 2, 3..and so on (up counter) and downward from 15 to 14,13, 12... and so on (down counter).
The up/down switching of the IC is handled by the D/U terminal (pin 5).
When this terminal is at positive level, IC works as a down counter. When it is at negative level, it works as an up counter.
When you finish wiring the project, turn power ON and operate the counter while looking at Figure 1 in previous project.
What number does it display first? It's 15.
Now press S1 to input the pulses and watch the LEDs, and observe how the counter works.