If your close to a guy and he does the "Z" thing it usually indicates they are on a TCP connection. AH tries to connect first using UDP, then if that fails (usually due to a firewall or router based internet connection) it switches to TCP.
AH uses a sort of lead-predictor to predict where planes are going to keep the flow smooth.
UDP is sending fire and forget packets of data, ie, if a packet gets lost then it is not resent, and AH fills in the gaps itself.
TCP is sending packets which are checked, confirmed, and if not received resent.
So, when you see a Z'er, its usually that one of the TCP packets has been resent, in the meantime AH has guessed a path fairly close but not exactly the one he took. The resent TCP packet with the right data arrives and is rendered and you see that little Z jump.
If the data is lost at too high a rate then the plane appears to freeze in midair or disappear.
The best solution is to get the person to setup their firewall/router to allow UDP traffic in from the AH server.
Intentional lag is relatively hard to do. The easiest way would be to start a massive download while playing, but that takes time, and in the heat of air combat you'd crash setting up the download