Comprehensive explainer on financial leverage (borrowing to invest), illustrating both amplified gains and catastrophic losses. Article uses South Korean market crash case study to show how margin calls and forced liquidations can wipe out portfolios in cascading market downturns. Key relevance for Thai retail investors using margin accounts, futures, or crypto trading—mechanisms widely available locally. Actionable warning: 1:10 leverage with 30% market drop can erase 60% of principal. Suitable for fintech platforms, brokers, and investor education content.