Palate Propping

The monster with More Teeth Than the Osmond Family is about to chomp on The Hero, who has only a stick/bone/pole for a weapon. What to do? Why, shove it into the roof of the oncoming mouth to prop those slathering jaws open! If the hero's really lucky, this might jab its palate hard enough to fatally penetrate the skull; at worst, it'll leave it gagging for a moment, then piss it off even more.

A common subversion is for the monster or animal to have jaw muscles that are powerful enough to snap bones or wooden planks in half, allowing it to break the item lodged between its jaws with ease.

This trope can also be applied to metal-toothed bear traps, despite their lack of a palate.

Examples of Palate Propping include:

Anime & Manga

Early on in Yu Yu Hakusho, Yuusuke uses a piece of wood to hold open the villain Gouki's mouth so he can Attack Its Weak Point.

Comic Books

In Tintin in the Congo, Tintin uses his rifle to prop open a crocodile's mouth when out of bullets.

Film

In Return of the Jedi, Luke braces the Rancor's jaws open with a large bone from the floor of its pit. This buys him a few seconds, but not much, as the Rancor's bite is powerful enough to snap it in two.
Subverted in the Doom movie: when one of the monsters gets its Palate Propped by a metal rod, it just clamps its jaws so hard that the rod pierces its muzzle and is forced out the roof of its maw, freeing it to go on attacking.
In Dragonheart, this trope leads to a Mexican stand-off, with the hero literally crouched inside Draco's mouth, his sword's point braced against the dragon's palate.
On the Wallace and Gromit short "A Matter of Loaf and Death", Wallace props open a crocodile's mouth with a French baguette.

Literature

Happened in The Cormyr Saga of Forgotten Realms during the duel of Iliphar Nelnueve and Thauglorimorgorus (the "Purple Dragon" who ended up as Cormyr's symbol), with a twist. Dragon breath easily removed the obstacle, but the destruction of a magic staff almost got his head blown off.
A variant in How the Whale Got His Throat, one of Rudyard Kipling's Just So Stories: the protagonist stops the whale from devouring pretty much everything by turning his raft into a grate and sticking it in the whale's throat.
The Adventures of Baron Munchausen's stories tell of the Baron getting swallowed by a whale along with his ship. During the council of all the people the whale ever swallowed on how to get out: I was chosen chairman, and the first thing I did was to propose splicing two main-masts together, and the next time he opened his mouth to be ready to wedge them in, so as to prevent his shutting it. After they get out, they leave the masts there to prevent the whale from swallowing more ships.

Live Action TV

Used against at least one snappy prehistoric beast on Primeval.

Video Games

A variant form in Hidden Expedition: The Uncharted Islands (casual computer game/IHOG) -- the gator has an item you need in its mouth, you need to find a jack to hold its mouth open long enough for you to grab the item.
In the first Simon the Sorcerer, Simon uses a stick to jam the jaws of a slavering Chest Monster.

Web Original

In V1E8 of RWBY, Yang stands in the mouth of a Giant Nevermore, holding its upper and lower beak apart with her entire body while firing blasts from her weapon Ember Celica down its throat.

I! Hope! You're! Hungry!

Western Animation

This was used in the Mr. Bogus episode "Bogus Private Eye". Bogus, with help from the family bulldog, tries to stop a thief that somehow invades the house. The bulldog tries to bite the thief, but the thief whips out a bone and sticks it between the bulldog's jaws.
In The Sword in the Stone Wart (in fish form) does this to the pike in the moat with a broken-off piece of spear. Merlin compliments him for using his head.
An episode of G.I. Joe had Gung-Ho pull this move in order to avoid getting chomped on by an alligator.
In the battle at the start of the Teen Titans episode "X", Red X does this to Beast Boy, who has taken the form of a gorilla. Beast Boy manages to free himself from it by shifting to a hippopotamus (which has a larger jaw) but this causes the girder he is on to collapse.

Real Life

There's a version of this that's used for giving anesthetic to an animal. A round tube with a hole is used to prop open the jaws and allow passage of the breathing tube down into the airway. The prop is sometimes removed once the tube is in place-just pass it backward off of the tube. Alternately, it may be left in and the jaw secured from the outside, just in case the animal happens to start to come around at any time, so it can't sever the tube.
There are form-fitted oral braces used for dental surgery, that prevent a patient's jaws from moving by reflex in mid-operation.