Packet hacking?

I saw someone on another forum, who suggested blocking packets. He succeeded in blocking "I got hit" packets, but he was still able to die. The things is, even though he blocked the packets being sent to the server, his client was still taking damage, and when his health dropped to 0 his client sent a "I died" packet, which he wasn't able to intersect. I'm a complete newbie at hacking, so I didn't really understand much. Later in that topic, they were talking about checksums, which I have no idea what they are. Just suggesting it here, because I think there are people who might be able to look deeper into this.

i was just thinking of PEing ROTMG yesterday too. I dont think its impossible or hard people probably are doing it for duping items.