How do you define momentum?
I think that momentum is not only having control and being in a good position, but being able to stay in a good position after that.
How do you obtain momentum?
By making good predictions, by using U-turn, or Substitute. Or other moves that force switches, such as Yawn. (very general I know -_-)
And Most importantly: What kind of POKEMON gives momentum?
This is a very interesting question, but I thin ka pokemon who has momentum must be able to force switches very easily, but must also be easy to counter. So that it can abuse moves such as Whirlwind, Substitute, or U-turn.
This is just my opinion.