Have you ever experienced while you are pedalling COGY, the slippy movement when you push the pedal with your full strength? It often happens on incline slopes. Also with a person with good strength, but with difficulty controlling the movement.
Every time the pedalling slips, in fact, the gear system is getting worn.
Gear system is a combination of 2 or more cogwheels.
Each cog is designed to engage to another, to produce designed motion.
When we put inappropriate pressure onto pedal, either intentionally or accidentally, this engagement slips.
Gear(A) spins without a cog (A) hooking a cog of another gear (B). In this process either/both of cog from gear A and B will be damaged.
It may be just a scratch, it may be broken teeth.
The system still works if the damage is only a small part of the system (only 1 cog out of many, etc). But, after getting damaged many times, on various cogs, system will become out of order, like this video.