They, like the Ornarans, possessed the natural ability to generate electrical discharges with their bodies, possibly due to the unusually strong magnetic field of their sun. It could have a mild effect of inducing a stasis in another humanoid, or it could kill.
Sometime in the 22nd century, the Brekkians helped the Ornarans survive a deadly plague by supplying them with the medicine felicium, which was available only on Brekka. By the 24th century, the plague was wiped out, but the addictive nature of the drug made it difficult to confirm the plague was cured. The Brekkians realized it, however, and exploited the situation by concealing this fact from the Ornarans, and continued supplying them with the drug.
Between the plague's occurrence and 2364, the economy of the Brekkians became entirely dependent on producing felicium; no other industries progressed in their society. They traded the "medicine" for goods made by the Ornarans. They therefore put their energy into improving the distillation process, increasing the drug's potency.
By 2364, the Ornarans had lost their ability to effectively maintain their ships, due to the effects of the drug. Transport between Brekka to Ornara, and therefore the economy of Brekka, came under threat.
The USS Enterprise-D visited Brekka to rescue the Sanction from its decaying orbit, which was also in the middle of a trade. Captain Jean-Luc Picard, citing the Prime Directive, did not intervene in the exploitative relationship the Brekkians maintained, but did not help the Ornarans fix their freighters, either, ensuring that Brekkian society would eventually need to become more independent. (TNG: "Symbiosis")