Jim Collins in Good To Great talks about momentum like it is a grinding stone in an old mill. A person/organization starts pushing on the millstone, and it moves a little. Push some more, it moves a little more. If you can keep up the continuous effort, the millstone moves faster and faster until you don't have to exert much effort at all to keep it moving.
The problem I have with this analogy is that the millstone literally moves in a circle. To be perfectly honest, you aren't going anywhere. On top of that, anything that gets thrown on the millstone gets ground down. To me, that isn't a moral boosting analogy.
Instead, I tend to lean toward another analogy: the snowball rolling downhill. Now, most people imagine a cartoon (probably loony toons) of a snowball that starts about the size of a marble and then gains speed as it rolls downhill. A cute picture, but not really true.
A snowball that rolls downhill takes a lot of effort to build up. Most of the time you need to pack a ball very tight and then push it slowly down the hill. It still takes effort to get the ball rolling, but at some point it starts rolling on its own. Then watch-out, because it goes where it wants to.
Unless, you can get ahead of the snowball and guide its' path. That means carving a trail, removing or avoiding obstacles, opening gates, and ensuring that the snowball moves in the direction you want it moving in; while still giving it the opportunities to gain mass and speed. Why do I say this. Well unlike the cartoons, most of the time, if a snowball hits a tree it stops and breaks apart. When it hits a fence, it piles against the posts and stays put. As a leader, you can pick the path that leads the ball past the trees and toward the open gate in the fence.
Of course that means you need to be far enough ahead of the ball to see the obstacles and deal with them. You can't stand there and look at the snowball rolling down the hill and think: "Well, we did it, now it's all on its' own."
So, leadership is continuous, and you have to mind the momentum. Your efforts help it grow, and keep it together, heading toward the destination you take it toward. Even when you leave, you need to make sure that the people who are taking over for you CAN take over the path clearing. Pick the right people, train them well, and ensure that your team knows the plan.