Yes using the gravity well of the planet to increase the momentum of the space craft. Don't know if they did it on Cassini but they will be doing it on the new pluto mission by slinging around Jupiter. Interesting thing though is that because of the conservation of momentum the increased momentum of the spacecraft cost Jupiter momentum. Not that you'd notice though
The moon is slowly slowing down the earth by the same process so by the time the moon breaks free of earths gravitational pull the earth will have slowed down so much that one day then will equal about 45 days now
A little bit off topic, sorry.