Let imagine observer and two objects (A and B). One object A with simple
movement can move only into one plane. The observer in own frame of reference
sees only this plane also.
Another object B with complicated movement can move in two planes and observer
can see only one of these planes.
The object A has position without movement for observer. The object B has a
movement. The object B has component velocity for each plane. However, observer
can see only one component velocity of objects B . The object A takes momentum
from object B after collision and objects A velocity may be higher than
inspected.
a. The observer can assume the object A has a huge mass. Or if observer knows
the object B mass then he can assume include dark matter for collision process.
b. If frame of reference base on law of momentum conservation then using all
know parameters easy to calculate another plane velocity component of object B
which is invisible for observer. This velocity component for observer has
imaginary character. From the other words, the frame of reference use law of
momentum conservation to readjust another object parameters.


A few words about movement without external forces. The object can rotate on own
center mass without external forces. However, if alternate simple and
complicated movement on object collisions then for observer frame of references
it may possible exchange rotation to translation movement.
Everything Is Relative.