Squats (front and back): Squats incorporate many muscles, and it may be difficult to contract them all evenly and at the same time. “Pre exhausting” the leg muscles before squats has a tremendous impact on the control one has over their muscles during a squat. That means using each muscle you intend to contract to perform a squat, before squatting. For example, I like to do jefferson squats, leg extensions, hamstring curls, straight leg deadlifts and calf raises before moving to heavy squats. I do not go to complete failure on the pre-squat movements, so by the time I squat, I have tremendous control over and awareness of each muscle involved, and am not fatigued, but thoroughly warmed up. This lets me pinpoint the muscles I want to emphasize when I squat, as well as making it easy to tell if one leg is working harder than the other. Lastly, I immediately know if my form is breaking down, at which point I adjust it or end the set. For emphasis on certain muscles, contract those muscles as hard as possible and independently of the rest of the muscles involved in the movement. For example, if one seeks to focus on the hamstrings, lower the weight by controlling it with the hamstrings until you can go no lower, maintain your awareness of the tension on the hamstrings, and contract them hard. The contraction will cause the hamstrings to shorten, propelling you upwards. The other muscles involved (quads, soleus, abductors, etc) will work less to allow for the hamstrings to perform more work. The other muscles will work, but contract less powerfully and act more as stabilizers to guide the movement produced primarily by the hamstrings. For glutes and hamstrings specifically, the movement ends before lockout. When you lockout, the muscles relax and stabilizers and joints bear the weight and tension. Therefore when emphasizing the hamstrings and glutes, you would not go all the way up and lock out, as that would relieve the tension on them. This is the same for a leg press; the quads are not stimulated when the knees are locked out.