I have no inside info on how the BF2 game engine works, and i am an amateur programmer (at best). So anyone who knows more please chime in and/or correct my mistakes.

If someone has a 50 ping to the server, that means roughly that their game is getting information from the server every 50 milliseconds. 50 milliseconds = 0.05 seconds. another way to express this: in one second the game will receive 20 updates. (1/0.05 = 20)

If the game only updated the screen when it got data from the server, your game would be running at 20 frames per second.
(ping of 25 = 40 fps, ping 100 = 10fps)

At those framerates, the game would be awful. So BF2 employs some sort of prediction/interpolation algorithm to draw more frames than it has data.

for example, I would guess that if you see a soldier running down the road, the game draws the soldier based on the info it gets from the server. Then keeps moving the soldier in the same direction and speed until your game gets another update from the server.

but what happens if the person controlling that character changes direction, or flops to the ground? you won't see it until 1) the person controlling the soldier's game sends its data to the server and 2) then the server sends it to your game.

But its not just you and the other player sending things back and forth, there are 62 other people doing the same thing. so 64 people are getting constant updates on the other 63 people. Thats a lot of data to send in real time.

The game uses UDP for transmitting the player updates (i think)
Unlike TCP, there is no error correction or delivery confirmation. (But UDPs simplicity also means it is faster than tcp -- thats why its used)

So, with UDP, a program specifies an IP address and a port number and sends off the message. It gets no confirmation that the message was received and has no ability to know if the data sent wasn't garbled somewhere along the way. It doesn't even know if there is anything 'listening' on the port and IP addr that it sent it to.

So your game is sending and receiving packets of data. What happens when the data arrives too fast for your computer to process it? It piles up in a buffer, but if the buffer gets full the incoming data is ignored. its gone. So if you get into a game situation that is very computationally intense (explosions smoke, etc), the program slows down --- messages start piling up in the buffer -- buffer gets full, messages discarded -- intense situation over -- start processing messages in the buffer again. Now the gap in the data does not occur until all of the buffered messages have been processed. So even though you see your framerate drop when the explosion/smoke happens, you don't experience the effect of the missing data until after that event is over.

I'm starting to get incoherent. honestly I'm amazed that BF2 works as well as it does.