u use ur abes......

you have to bend your knees! you have to bend your entire body! the point is to move in a doplhin motion....your whole body!

When you bring your arms down, follow the motion with you whole body. start at practice with just doing the dolphin kick with your arms above you head. go about 6 in under water and follow the motion with your whole body and it will come with practice

u have to use your hips not your shoulders

use your hips! try holding onto the edge of the pool and keeping your toes touching, hips moving up and down, and your knees straight. If your knees bend a little bit thats ok but try to make the whole movement from your hips.