Yes, its either using IDLE on nokia's side, or polling at very high frequency on nokia's side. On the phone itself it pushes as soon as new mail is detected, but the *phone* does not poll at all.

You get both faster response time to new messages and lower battery drain.

When my phone is sitting on my desk, it will buzz with new mail instantly, while evolution logged into the same imap account and set for a 5 minute poll obviously does nothing until the next 5 minute check. On the flip side, if no mail comes in for an hour (as if) then Evolution will have polled 12 times for no data, while the n900 will have done nothing at all.