Weightlifting is a leg sport. If you've been Snatching and Clean & Jerking for a couple years or more that may seem obvious, given how often and how heavy weightlifters squat. However, the way many lifters move you'd think them unaware of the importance of their legs in a lift.
Setting up the legs to explosively drive the barbell away from the platform is not a natural movement. Learning to do it effectively takes time and a downward reorientation from the default habit of arms being the primary interface with the the environment. Early on new lifters are taught to take their arms out of the pull so their back, and especially their legs do the work. But it may take thousands of reps to start feeling their legs at work in a full-speed lift.
There are some drills that can help speed the process of learning to feel the legs in the lift. One of the first drills I use is the Shift & Stand Drill. This drill teaches slowly what you will do much faster in the actual lift. Many physical skills in other sports are first taught slowly and gradually sped up. It is a time-tested way to acquire new motor skills, but for some reason we in weightlifting have accepted the orthodoxy that "you can't teach the double knee bend," as if gospel. The "double knee bend" or "scoop" can be taught. I do it every day. This transition is left to luck or fate or innate athletic ability by far too many coaches. It should be worked and understood, as this can speed up the process of feeling the legs working in the lift.
The Shift & Stand drill teaches what I believe to be the most difficult part of the pull to master: the transition from bar at the knees with torso angled forward over the bar, to the power position with torso just behind the bar, to vertical drive with the legs. It also teaches the proper mid-foot, heels-down balance point from which to drive up on the bar.
After the Shift & Stand Drill, I sometimes use a jumping drill I learned from Sean Waxman of Waxman's Gym. (My good friend coach Don McCauley may blanch at using the word jump, but every now and then, understanding the context and correct application of the concept, even stubborn old coaches like me can make use of heresies that can help our lifters) Using the Shift & Stand Drill before this jumping drill makes learning the jumping drill much easier to perform and master. Between the two drills, most lifters will start to feel the "quad punch" from the thighs that drives the bar up. They will begin to feel the correct direction of the drive and the power of the legs to accelerate the bar. The way I teach this drill, the lifter will land flat-footed no more than one-and-a-half inches behind a line drawn on the platform. Too far back is bad, forward over the line is worse.
Something to note about these drills or any drill; they are solutions to problems that have the potential to produce new problems if not applied judiciously. Every drill can introduce new technique shortcomings if overused or used too long. The Shift & Stand Drill can cause the top of the second pull to be too mechanical and slow. I use it only as long as necessary to get basic mechanics down, then try to get away from it. The Jumping Drill can cause the athlete to dramatically overemphasize the top of the pull, causing them to linger at the top of the pull and ruin the timing of movement under the bar. This can be corrected with lifts from high blocks or No-Feet Snatches and Cleans, but the more ingrained the problem, the longer and harder the effort to fix it. That said, the Shift & Stand Drill and the Jumping Drill can go a long way toward getting the athlete to really feel their legs at work at the top of the pull.
Weightlifters suffer through so much squatting in training that you'd think THAT was the sport. It isn't, but you can't be a great weightlifter without powerful legs. Nor can you be a great weightlifter if you don't know how to get that leg strength into your pull. So if you want huge weights to go up, you have to start thinking down.