Pliable Wood + Elongated Shaft = Easily Bendable/Breakable Spear
This video demonstrates step by step on how a Shaolin Monk breaks a spear. Look at how easily the shaft bends as he applies his weight to it. This is misleading as a deadly weapon, because if a spear did this during combat, it would be rendered useless very quickly. Listen closely to the 'hollow' sound of the wood hitting the ground in the video upon breakage. Dense wood, such as that of a real spear, does not sound this way.
It doesn't stop with mere bendable spears.
This video demonstrates a Shaolin Monk bending rebar with his neck as well. One key thing to realize isn't his many years of perfecting his mind and body in a temple - rather his hopping up and down to create a bending point for the rebar.
Penn & Teller: Bullsh!t (Episode 109 / Self-Helpless) actually show quite well the breaking of an arrow, the bending of rebar, and even walking on hot coals - and how the average person can achieve this without needing to practice within a Shaolin Temple.
The word of the day is Buckling.
Buckling is characterized by a sudden sideways failure of a structural member subjected to high compressive stress, where the compressive stress at the point of failure is less than the ultimate compressive stress that the material is capable of withstanding.
Here are some videos of 'regular people' bending rebar with their necks.
Here is a video of a 'regular person' breaking a spear with his neck.
Most telling of all, is this "Qi Gong Master" bending 3 spears at one time (notice how he pushes the tips with his chin into his breast bone.