The "catapult" or "trampoline" effect is bad pseudo-science. You don't get more force for free just because the strings are looser. Energy is always conserved. A tight stringbed deforms less, but it takes correspondingly more energy to deform. This energy is released as the strings return to their normal length. In principle this means that power should be the same regardless of tension: it would purely depend on how much (kinetic) energy the player "created" through his hitting action. In reality it's a lot more complicated than that, because an efficient hitting action transfers a higher proportion of energy into the shuttle, rather than elsewhere (e.g. back into the arm). Depending on the player, lower or higher string tension will be more efficient. I suspect the response time of a high-tension stringbed is shorter, and that this goes well with a high racket head speed. It might be that with a lower tension instead, the shuttle would already have left the strings before they finished bouncing back into place -- and therefore more energy would be wasted. Conversely, it may be that at slower racket speeds, the shuttle stays in contact with the strings for longer, and a slower stringbed response is more efficient. It's also possible that with higher tensions, a greater amount of minimum force is required to overcome static friction, before the strings will actually start to move. Regardless, there is no magical "trampoline" effect that is always better at low tensions. There is only energy stored in the string deformation, and the same energy released when the strings rebound (minus some losses, of course). Cutting through all the theory, you can easily feel the difference. When you get less power and feel more resistance to your arm during the hit, your string tension is wrong.