To be specific, it's the 2nd part of the 2 part pair of roll-n-time and >diff1 shares
Roll-n-time means you ask the pool for work and can make multiple work items with the one work item you get from the pool
Typically it seams around 10 or 20 work items per single work request for someone in the 1GH/s to 2GH/s range.
Thus you are asking the pool for 1/10th or even 1/20th of the information, thus your requests to the pool for work are also 1/10th or 1/20th of the bandwidth
... and the pool has to do 1/10th or 1/20th of the getwork work for you that it needs to do without roll-n-time.
However, you still find the same number of shares and thus have to send back a share for each one to the pool.
Higher difficulty shares means you are telling the pools you have done more work, so you get paid for more work, each time you send a share back to the pool.
An example would be 2 diff shares.
A 2 diff share means you will only find half as many as you would if you were looking for 1 diff shares.
Since you only find half as many, they get paid twice as much.
As a result, you are also only sending half as many shares to the pool - so again you are using half as much bandwidth sending shares to the pool and the pool only has to handle half as many shares processing ... from you.
However, there is of course also a down side to >1 diff shares, it increases your variance.
However, anyone mining with a high hash rate is already mining with a lower variance than people with a low hash rate - so it's not really a negative thing in that respect - as long as the difficulty shares you are getting are appropriate for your hash rate.
See the posts by the BTC Statistics Squad for details about what difficulty shares are appropriate for your hash rate
(of course I am referring to people like organofcorti and Meni