Part of the problem (unless you are doing some folk forms) is that part of your training is to find the least amount of muscle to achieve an effect. So while a beginner will engage lots of muscles at full intensity, a more experienced dancer will fire just what is needed for just as much as needed. This is what gives the look of grace and relaxation. Yes, shimmy too - you should be relaxed when shimmying (thighs and butt should slap and flap).