What is the mechanism by which passive immunity works?
I commented on this question, but the OP's response prompted me to think again. Here is the graph from the document that the OP linked to:



Clearly what is confusing is that the parameter referred to as 'antibody level' rises quickly, but not as a step. In terms of the x-axis, vaguely labelled as weeks, it looks as if the level of antibody continues to rise over several days at least. The OP attempts to read into this an effect whereby the injected antibodies somehow stimulate the immune response in the recipient.

My knowledge of immunology isn't extensive enough to rule that out completely, but I don't think that is what this sketch graph is illustrating since the levels of these antibodies is shown as falling to zero due to destruction and excretion. This indicates that what is being graphed is just the injected antibodies.

In the absence of some real data for the time-course, my explanation for the rising phase illustrated is that this is showing levels of antibodies in the blood. However the prophylactic use of injected antibodies will often involve an intramuscular injection near the site of an injury. Here for example is a quotation from a recent review on the use of recombinant immunoglobulins (RIG) against rabies:


  Because vaccination needs time to induce a humoral response, application of RIG immediately after exposure is important to inhibit viral spread, even when accelerated vaccination schedules are applied. As much of the RIG dose as anatomically feasible should be injected into the area around the wound, and the remaining volume then given intramuscularly at a site distant from rabies vaccine inoculation. Isotope-labelled HRIG, injected around simulated bite wounds, diffused readily into surrounding tissue and about 25% was still detectable around the injection site 24 h later.


Thus the level of antibodies in the blood will continue to rise as they slowly enter the circulation from the site of injection.