Essentially the player character(s) can change direction 180 degrees at dead run in an instant and feel no vertigo, delay or any ill effects.

A real person subjected to this would likely suffer from muscle damage (sprain) and/or concussion. The effect would be somewhat similar to two people running as fast as they can at each other, the collision being the point at which the game person had changed the direction they're moving in 180 degrees. Note that in the collision the two people take no precautions or anything else to lessen the impact.

Older games had this simply because it takes too much processing power to simulate human motion.