Something that has a lot of momentum is hard to stop. A bowling ball. An ocean liner. A person who will not allow themselves to be derailed.
Physically, an object of any amount of momentum could be stopped in a very short time, if enormous force is exerted. And the same could apply, in theory, to mental affairs. But there's a beautiful symmetry: the momentum you could gain in a day, you could lose in a day. The momentum you build in a year will not be lost due to one horrible day, or even a week or month.
That is the value of increasing your own: if you keep growing your momentum every day, you become harder and harder to stop -- even to yourself. Once you have momentum, it is easier to succeed than fail.
You know when momentum is NOT there. You know and feel it when suddenly you get momentum - there is a reversal in the game (in sports, in work, ...).
How is momentum created in a team? Someone in an uncertain situation takes a certain & positive stand in their body. Like -- Steph Curry shoots a 3-pointer, with absolute certainty -- doesn't even look at the ball after making the shot and knows it's going to go in.
How do you keep momentum going when getting punched in the face, as you invariably will? By relying on a strong Emotional home. What messes up momentum: Fear.
"Hoog" in Estonian.