Many relationships connect you to NPCs controlled by the GM.
When you try to use one of these relationships to solve a problem, your tactic is your relationship ability. You can’t simply go to the supporting character you have a relationship with, stick them with the problem, and expect to see it solved.
If you succeed, the supporting character helps you solve the problem. If you fail, they don’t. As with any ability, you must still specify how the NPC goes about overcoming the story obstacle. Calls on relationships are almost always simple contests.
In crucial situations, it may seem dramatically inappropriate for you to solve a problem indirectly, by working through others. Your GM can expose the supporting character to serious risk. If the character dies or otherwise suffers a change of status that renders them useless to you, you permanently lose the relationship ability.
Before putting supporting characters at serious risk, your GM should make sure the players understand the magnitude of the possible consequences.
When supporting characters undertake significant risk, the supporting character may suffer a consequence of defeat commensurate with the level of the defeat in the contest. Or it may simply be your relationship that is damaged or destroyed.