Have we, if you will, an immovable object, represented on a plane as a point.
The immovable object is then acted upon by an unstoppable force. The unstoppable force is a vector with an infinite length, but we can represent it with a finite length because we have a zoom out button.
Since our object is immovable, it has to counteract this unstoppable force with its own infinite vector, in the opposite direction.
And then on top of all this, Undyne suplexes the object.
Undyne’s suplex vector pulls the immovable object in a direction different than the unstoppable force does, meaning that the object can’t counteract it, as it’s already counteracting the unstoppable force.