I see momentum as putting yourself in a favorable position over your opponent. Forcing them to respond on what you do instead of the other way around. I see it as deaply conected with prediction, and will mention how i look on it in general and a few moves usaly conected to momentum.
I consider a favorable position as when you can do alot of damage to your opponent while they can't do much back in return. When you find yourself in that position through the match, you have the momentum on your side. The damage you do, don't have to be direct damage but anything that hurts your opponent in a way. There are two main ways of achiving that.
Offensive Approach: Use your offense and speed to threat your enemy doing lots of damage before they can.
Defensive Approach: Use your defence to make your opponents atempts of hurting you minimal while slowly finishing off or setting up a favorable condition for you.
Ofcourse this isn't black and white, most things are somewhere in between. offensive threats need a form of defence to be able to survive. Otherwise they can't switch in and their usability is hindered. Same with defensive pokemons, many primarly defensive pokemons also have a form of offense or even set up to make a sweep once its stongest counters are gone. Even a defensive pokemon have to threat somehow be it through residual damage, direct attacks, crippeling enemy pokemons through status and similar, or setting up something.
When it comes to keeping momentum prediction plays a huge role. When you are in a favorable position, your opponent have to act somehow toget back on even ground or even retake the advantage. By outpredicting his moves you can limit those atempts. A counter won't change much if you can predict it and either choose a move that can help deal with it either on its own, or making it weakened enough to lose its use later in the game. Or switching to something to deal with the expected counter.
Momentum moves often mentioned such as U-Turn and Baton Pass work in a way as they remove a part of the prediction needed. Instead of having to predict what they will switch too, you only have to predict wether they will switch or not and then bring out an apropriate pokemon. Baton Pass comes with the risk of wasting a turn you could have been attacking with while the more powerfull U-Turn makes it all less risky as it isn't just a switch but deals damage aswell. To note is that neither of theese moves give momentum on its own (arguably U-Turn can by posing a threat on Bug weak pokemons).
Moves that does give momentum however are crippeling moves. Spore threaten to incapacitate any pokemon thats not a sleep absorber. Memento also comes to mind. Forcing your opponent into an unfavorable position where they gott respond a certain way.
I'm not saying this cap need spore and memento, there's plenty of other moves it could choose from, or it could focus on gaining its momentum through offense rather then suportive moves.
Also want to point out U-Turn is by no mean nessesary to gain momentum, it would help with transfering momentum to your team easier but isn't cut in stone. I partly against U-Turn myself as I feel rather explore how we can transfer momentum to our team by other means.
By any means want to make it not take a sweeper role, or a choiced U-Turner.