It's not really changing the keys.  Poser uses a continuous spline which adopts over the entire spline when new keys are added or keys change in value.  So the spline changes over frames between keys according to the spline interpolation, yes.

As Phil stated, that's the way to go

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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