The dorsal and anal fins of fish interact with the tail fins to produce higher thrust and efficiency. We focused on thin elongated dorsal fins, like those of jackfish. By applying a mathematical method to parameterize the presence of elongate dorsal/anal fin, we can isolate the effects of fins individually while the side projection area between fins keeps constant.We used a tuna-inspired fish model with variable fin sharpness to study the interaction between elongated dorsal/anal fins and caudal fins. We found that the performance enhancement is stronger than previously thought (15% increase in swimming speed and 50% increase in swimming economy) and is governed by a three-dimensional Dorsal Fin-induced Crossflow that lowers the angle of attack on the caudal fin and promotes spanwise flow. Both simulations and multi-layer Particle Image Velocity that the crossflow stabilizes the Leading Edge Vortex on the caudal fin, similar nohow wing strakes prevent stall during fixed-wing aircraft maneuvers. Unlike other fin-fin interactions, this mechanism is phase-insensitive and offers a simple, passive solution for flow control over oscillating propulsors. Our results, therefore, improve our understanding of multi-fin flow interactions and suggest new insights into dorsal/anal fin shape and placement in fish and fish-inspired vehicles.