Parivartana is worth reading alongside the whole class of combinations that depend on two planets being connected, because it is generally regarded as the strongest form that connection can take.
Jyotisha recognises several ways in which two planets may be associated: conjunction in one sign, mutual aspect across the chart, one planet occupying the house the other rules, and exchange, in which each occupies the sign the other rules. Exchange is treated as the most binding because it is symmetrical and unavoidable. Each planet sits in the other's territory and neither can act without involving the other, so the two significations are described as fused rather than merely in contact.
The tradition classifies exchanges by the houses involved. An exchange between two auspicious houses is called by names meaning growth or increase; one involving a dusthana is treated as a misfortune and given a name to match; and an exchange between a dusthana and an auspicious house is the contested middle case where authorities differ.
Because exchange is defined purely by sign lordship it can be checked mechanically, without any judgement, which makes it one of the few things in the system two practitioners will always agree is present. What it means remains a matter of interpretation.