It activates Triangle()
with a parameter of 2.
Here is the new picture. You may be a bit uneasy about
something called "Triangle" being active multiple times.
But this is perfectly fine.
Think of each activation as being a "clone" of Triangle()
that has been given its own little task.
What does the activation Triangle(2)
do?
int Triangle( int N ) { if ( N == 1 ) return 1; else return N + Triangle( N-1 ); }