But this is static training. If we're practicing dynamically, or if we're out in the world, why did nage allow uke to get a strong grip in the first place?
Two points, echoing what Jon said.
First, static versus dynamic is a false dichotomy. The point can be expressed in innumerable ways: dynamic is in the static and static is in the dynamic. In-yo. Potential energy resolves to kinetic as kinetic resolves to potential. The orientation of a moment (rotational/torsional stress in a structure) lies in the same path of the resulting momentum when it is released and the stress (potential) becomes kinetic (motion)- and vice versa -- you can absorb/deflect kinetic through structure -- ("To a point, Lord Copper, to a point.") But quibbles aside, the distinctions of expression are -- Tomato::Tomahto.
Second, the grip is in the context of
attack, It is premised on the assumption of the presence of a weapon or strike -- say, a grip to stop or delay the drawing of the sword or use of the knife -- or prevent atemi. Again, this is not static versus dynamic -- but to study using structure to defeat (or more usually moderate or better direct) a dynamic that is already present. Conversely, when the student's grip is ineffective or lacking, I illustrate with a tanto or wakizashi thrust -- and then the grip gets intuitively
correct. At the same time, it is the building block -- or sandbox, if you will -- best used to observe means and effects of various orientations of stress and movement on structure, because when when things become more fluid -- the nature and form of connection to the attack becomes less fixed and obvious.