Besides the common ways to create card relations, the Mind Map gives you additional possibilities to do that as well. This applies only to parent and child relation. In the Mind Map view, cards have connections shown as lines:
Single card connected to the root node.
Subcard connected to the parent card.
Parent card connected to the root node.
When the subcard and the parent card are present on the Mind Map, their lines visually merge.
Adding subcards in the Mind Map view
You can create a parent and child relation by dragging a card and dropping it on another. The dragged card will become a subcard.
When the parent card is missing in the Mind Map view, the subcard is connected directly to the root node. Note that when you add the parent card, the lines are redrawn to show the actual card relations.
Now the parent card is connected to the root node and the subcard is connected to its parent.
Even if the card has more than one parent, only one of them is the default parent. The connection visible on the Mind Map view is the one between the child card and the default parent card.