[the_cake]

Yea it's hard to explain, let me try again.

When you play any online game, the server sends you "updates," which are basically a single pieces of data telling you where all the "stuff" (other players, projectiles, sgs, whatever) are on the server at a single point in time. A fairly bad server will send you 33 of these updates every second, maybe less depending on your connection.

So if the server is only telling you where stuff is 33 times per second, how do you get more than 33 frames per second on your computer?

That's what interp does. It saves up more than one of these updates, then tries to guess what happened between the updates. So if an update says a player is at point A, and the next one says player is at point B, it will guess that in between those two updates the player was somewhere in between points A and B, and render that as a frame.

So with 3 updates, no interpolation, you would see:

update 1 from server=frame 1 on video card
update 2=frame 2
update 3=frame 3

With the same 3 updates, interpolation on (aka not interping), you would see

update 1=frame 1
(engine guesses what happens between updates 1 and 2)=frame 2
update 2=frame 3
(guesses between 2 and 3)=frame 4
update 3=frame 5
(guesses between 3 and 4)=frame 6

That's twice as many fps in the same amount of time. And it can guess as many times as it needs to between frames, so you could have 1000fps and 10 updates/second, although the amount of guessing would make it really inaccurate.