As in many approach/repulsion dynamics in nature, the passage of the 12th Planet through the solar system has many factors at play, at once. In psychology, there are approach/approach conflicts, where an individual is pulled in two directions at once, both equally attractive, and becomes paralyzed. There are likewise avoidance/avoidance conflicts, where an individual is caught between two situations he would like to avoid, and likewise becomes paralyzed. A third conflict is approach/avoidance, where an individual is both attracted to approach and trying to avoid a situation, and thus dithers or moves slowly in a direction. The 12th Planet is caught, not because of psychology but because of gravitational and magnetic aspects, in an approach/avoidance conflict with the Sun. Thus:
When the 12th Planet rides mid-way between its two foci, the Sun and its dead twin some 18.74 Sun-Pluto distances away, it is in an approach/approach situation, and barely moves during the majority of its 3,657 year cycle.
When it is within a few years of a passage of one of its foci, breaking from its mid-point position and picking up speed in an approach of one of these two suns, it increasingly becomes a non-conflict situation, approach only, as the 12th Planet is pulled by the gravity of the sun it is closest to, yet far enough away from that sun that a repulsion force has not yet come into play. It picks up speed, this speed adding momentum, in a virtual straight line approach.
When the 12th Planet nears one of its foci, the repulsion force comes into play. There are other factors that influence close contact between large bodies, but the repulsion force is dominant. The 12th Planet slows, increasingly, as what we have described as a fire-hose of gravity particles from both the Sun and the 12th Planet are pointed at each other, butting into each other and pushing the gravity giants away from each other. This is a minor factor at first, reducing the increasing speed of approach. Then it reaches the point where the approach is actually diminishing, losing momentum gained before.
The 12th Planet deals with this situation by sliding sideways, away from the main point of these gravity particle spurts, which occur at the Sun's belly, the Plane of the Ecliptic. It dives south, still approaching as it continues to be attracted due to gravity. That it pierces the Plane of the Ecliptic, rather that skidding along the southern part of the Sun, is due to taking the path of least resistance. During the 3 months it takes the 12th Planet to traverse the solar system from one side of Saturn's orbit to the other, it has placed itself on a line some 32 degrees below the Ecliptic due to its slide sideways to avoid the repulsion force.
Its angle of approach is still from south to north, the dictates of inertia and momentum stating it would continue in this direction. As this line of approach brings the 12th Planet into conflict with increasingly strong gravity spurts along the Ecliptic, it must both turn into these spurts to turn south, as well as slow its forward momentum, so the path of least resistance is to move north, past a given spurt, for avoidance. Thus, it crosses the Ecliptic, jerking northward during each encounter with a spurt, while still moving toward the Sun in general due to gravity.
The overall effect is for a rapid pace toward the sun in the last months, with a dive southward and a slowing in the last weeks, with an even slower passage as it crosses the Ecliptic in the week of passage. It seems, almost, to hover in the sky as it crosses between the Earth and Sun, moving slowly as the horrified populace watches.