The dolphin motion is how most other aquatic creatures move through the water. It’s a motion that requires you to feel the water and work with the water more so than flutter kick or breastroke kick (frog kick). Keep your legs together, start the up and down motion in your upper abs, and let the motion propagate through your body until it finishes with the lower legs. When you’re dolphin kicking easy you’ll feel it mostly in your abs and low back. When you dial up the intensity, you’ll start feeling it more in your quads and glutes. The strong quad activation only comes when really trying to “snap” the lower legs at the end of the motion. Dolphin kick works best when already moving at speed, so it’s very hard to get started from a standstill. A novice will still have a large amount of movement from the abs up. As you gain proficiency the upper torso and head will remain more still and only the lower body will undulate. A beginner mostly needs to make sure they are not doing accordion legs and is instead using all the muscles from the abs down to produce propulsion.