like others have said, it took me years to perfect the form. dont do what you see others doing, you gotta feel it.
ex: I got long arms, so I tend to get on an elevated platform and start with the bar almost touching my toes, then pull up (through the glutes and hams) to about my knees. any higher and pressure goes from glutes/hams to back. But guys with shorter/normal length arms probably need to go up past the knees and only go down to the ankles. Big mind-muscle connection needed here.