Exactly what cases does the gcc execstack flag allow and how does it enforce it?

I have some example code here which I'm using to understand some C behaviour for a beginner's CTF:

// example.c

#include <stdio.h>


void main() {
        void (*print)();

        print = getenv("EGG");
        print();
}

Compile: gcc -z execstack -g -m32 -o example example.c

Usage: EGG=$(echo -ne '\x90\xc3) ./example

If I compile the code with the execstack flag, the program will execute the opcodes I've injected above. Without the flag, the program will crash due to a segmentation fault.

Why exactly is this? Is it because getenv is storing the actual opcodes on the stack, and the execstack flag allows jumps to the stack? Or does getenv push a pointer onto the stack, and there are some other rules about what sections of memory are executable? I read the manpage, but I couldn't work out exactly what the rules are and how they're enforced.

Another issue is I think I'm also really lacking a good tool to visualise memory whilst debugging, so its hard to figure this out. Any advice would be really appreciated.