It's not a trick, it's combined usage. It's like having a car and driving alone. You'll not efficient. Pack in 5 people and then it's much better than all five people wasting gas.
Check the article at the Official Google Blog:
Gmail: It’s cooler in the cloud: "(Cross-posted on Gmail, Green and Enterprise blogs)[...]
If you’re more of a romantic than a businessperson, think of it this way: It takes more energy to send a message in a bottle than it does to use Gmail for a year, as long as you count (PDF) the energy used to make the bottle and the wine you drank.
We ran a similar calculation for YouTube and the results are even more striking: the servers needed to play one minute of YouTube consume about 0.0002 kWh of energy. To put that in perspective, it takes about eight seconds for the human body to burn off that same amount. You’d have to watch YouTube for three straight days for our servers to consume the amount of energy required to manufacture, package and ship a single DVD.
Posted by David Jacobowitz, Program Manager, Green Engineering and Operations"
[via Official Google Blog]