The glutes and quads are stronger than your hamstrings. Should use those more which I imagine is where the mud thing came from. Use whatever visualization you need to have a smooth stroke throughout. scraping mud, log rolling, sliding on ice, whatever. The thing is when you're on a long enough ride you'll know when you're not pedaling smoothly. It will be almost all up and down and little to nothing on the top and bottom (aka pedaling squares). Just don't do that and you're closer to a smooth stroke. GL