Example of how client sided hacks are good:
Client says to you: There's a guy on the screen.
You say to client: Shoot him, but I'm modifying you so you hit his head only.
Client says to server: I shot his head.
Server decides: Headshot? Guy dies. +1 for client
#48362
Server says to client: +1
Client says to you: Headshot!
Example of how client sided hacks are bad:
Client says to you and server: You (
#48362) got shot a lot by some guy.
Server decides:
#48362 is at 10 HP.
Client says to you: You're at 10 HP.
You say to client: I'm gonna use Cheat Engine to set my HP to 100.
Client says to you and server:
#48362 got shot by a guy again.
Server decides: Client
#48362 was at 10 HP.
#48362 dies.
Client says to you: Dead. Respawning in 3 seconds...
You say to client: FUCK YOU!