<p>Continuous flapping propulsion in bioinspired underwater robots often leads to high energy consumption and limited endurance during low-speed cruising. Inspired by the crest stay behavior of manta rays, this study proposes a propulsion method that combines a flexible fin design with a control strategy. First, we develop a two-degree-of-freedom flexible pectoral fin that reproduces distributed flexibility and supports independent flapping and twisting. Then, we propose an improved central pattern generator model with an explicit crest stay mechanism. The mechanism is embedded into the phase oscillator through a crest stay condition and equation. Next, we conduct systematic pool experiments to evaluate how crest stay time affects performance under different flapping amplitudes and frequencies. The results show that an appropriate crest stay time increases thrust and reduces energy consumption compared with continuous flapping. Moreover, lake experiments with a full-scale manta ray robot quantitatively validate the energy-efficient effect of the crest stay mechanism, achieving a 31.6% reduction in energy consumption without compromising cruising speed. This study is the first to provide practical evidence of crest stay effectiveness in large scale robots, highlighting its importance for the energy-efficient design of long-endurance underwater vehicles.</p>