Timothy Walters Kleinert
I can see a few different ways one can blend/flow. The first is with physical momentum. This is big in popular Aikido.
The second would be... I guess you could call it mental/psychological momentum. I think you see this more with sword work, and not much in popular Aikido.
Third would be external structural flowing/blending. This would be following the weak lines of the body & form.
And the last would be internal structural flowing/blending. This would involve "feeling" where a person's structure was weak, or how they were distributing internal forces, and flowing/blending with that. For example, if my teacher feels a weakness in my structure, he can "attack" that weakness and overpower me on a "strong line".
I will posit (have regularly posited) that it is not in fact four ways but one way with the four aspects that you address.
1) NOT physical momentum (m * v) -- but angular momentum
(I * ω). Not the same thing. One is linear billiards and common sense 2D vector sums -- the other one is 3D or 4D "english" and spooky and counterintuitive. Works for swung swords, articulated bodies and pool balls all the same. Angular momentum is conserved -- bodies forming a system conserve angular moment as to the system such that alteration of one (almost) instantaneously alters the angular momentum of the other(s).
2) Instantaneous perception of the actual state of angular momentum can be performed analytically, but not by ordinary human brains. Intuitive sense-pattern building is the only way to grasp it in a concrete immediate sense.
3) Transmission of angular momentum through an articulated body can make a structure more coherent (locking up) or less coherent (destablizing) depending on its application and the situation.
4) AND The flip side of 3, management of angular momentum through the articulations of the body can prevent structural decoherence (rooting) or use decoherence advantageously (void or lightning).
This is the mechanical basis for what we are talking about. The psychological factors can take any shape (mythical, poetic, physical or pseudo- physical) that happens to work for the imagery of the practitioners. The practice process in grasping the use of angular momentum is fourfold along these same lines:
A) building fundamental sense-patterns,
B) heightening sensitivity to make yet more subtle sense patterns
C grasping the nature of dynamic articulation of the body in sense-patterns,
D) grasping the nature of systemic connection in sense patterns
(iterate ad infinitum)