When a trendline breakout point acts as a good supply zone, it means that the area where price broke above a trendline later attracts strong selling pressure. After the breakout, early buyers book profits and trapped sellers look to exit at breakeven, creating fresh supply near the same level. If price revisits this zone and fails to sustain above it, sellers regain control and push prices lower, confirming the zone as a supply area. This behavior shows that the breakout lacked strong follow-through, and traders often use such zones to look for short setups with clear risk defined above the supply zone.