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.

People have found out how to duplicate, thats for sure. Now the question is how.

What kind of coding would be needed to understand packet editing?

Also, I heard that Weedman and the Syndacite (Spelling might be off) used Packet editing to use their teleporting/god mode hack.
(Heard that from Weedman himself.)

The sided-server handles the user's HP as well as Their Items (inventory, equipped), their drops, their mp, and much more. It means it cannot get hacked, using a normal server, without getting kicked or false positive (like he froze the hp but still died).