I believe it's mostly skill...and some people modify their clients to enhance the "skill" if they arent good enough.
Over the years, people have modded the client to take away jump animations, some have changed shadows, some have had packet logs to place the shadow dot where the person will land, etc...
When you jump, CO sends your landing coordinates to all clients, except we all know a CO jump doesn't travel as fast as packet of data shuffling across the internet. So technically your client, and everyone elses clients sees where your going to land, but you and everyone else is seeing you jumping. People in the past, and probably still now, have found ways to disable the animation and just show you as landed, so in essence, you don't jump on their screen, you "teleport".
Ex.
You Jump from point A to point B.
Before you see your jump, or anyone else does, your client sends to CO "I'm jumping from point A to B".
CO sends to everyone "Player XXX is jumping from point A to point B".
Each client recieves this info and tells the graphics engine to animate you jumping from point A to B.
What if the client is hacked not to show jump animations? it still would need to show where you landed.
Instead it shows you instantly going from point A to point B.
On your client, it still shows you as in mid-jump, since the jump animation is timed.
The person fastbaldes/scentswords the "teleported" you, who is standing still.
Your prolly wondering..."why would I be standing still?" that's because your in mid-jump on your own screen, and if you click anywhere else to move, your client won't send that packet out until you have actually landed on your client screen.
Now what some people have done, is taken this to the next step.
They took out the middle man....YOU.
They packet log the Point A to Point B packet, and then send or click, depending on the sophistication of the bot, at the location or degree of direction that your victim will be landing.
PS. sorry for such a long post. I'm no expert, but hope this helps =)