This is not 100% true, it's only a thing in specific cases.

The part of the intestines directly connected to the anus and next to the vagina is called the rectum.

Normally there really isn't that much poop in the rectum unless you need to shit bad or have just shit. It's like a buffer your body fills up when you are preparing to go poop, the shit is stored before the rectum and you have muscles that push the poop into the rectum. During the shit the rectum gets squeezed tight and is thus mostly empty. Filling the rectum usually induces the feeling of needing to poop.

However in cases of chronic constipation, some dried out clump of poop can get stuck inside the rectum. By manually pushing out this clump, the rectum can be cleared and normal function can resume.

Usually in cases of constipation the body has trouble even filling the rectum. So squeezing it will do nothing and is probably not recommended. Your body is also really good at squeezing the rectum, with specific muscle groups which serve only to do this one thing.

Also: Don't ever take medical advice from the internet, talk to an actual medical doctor, your personal doctor if available.