As I understand it, you tune an antenna array (typically a vertical colineanar of some type) to a higher frequency. When you feed it a frequency below it's design signal, the phasing of the elements causes the main lobe to tilt down (I may have this backwards, ie lower tune for down tilt). The effect helps fill in low spots, or improve coverage closer to a very high antenna. Tilt of 4-5 degrees are common. You don't see this much below UHF though. If the coverage area is on one side of the antenna, then a simple tilting of the antenna accomplishes the same thing
I'm sure that I'll be corrected if my memory is off....
Are you asking about the propagation of E-tilt vs mechanical tilt?
I'm a performance engineer for a rather large wireless carrier. We actually have remote e-tilts in our UMTS system now. It's the coolest thing for an RF Engineer. I can sit at my desk and add/reduce the e-tilt on our cell sites. What's even cooler? Sitting in the far field of a sector, connected to our internal network via UMTS aircard w/ VPN, and moving the tilts on the antennas until the overshoot/interference, etc is gone, right before your very own eyes!