Is there a one true way (OTW) to lift a barbell off the ground? Probably not. When human biomechanics and sport technique are involved there will always be some freak who defies conventional wisdom. Are there some best practices? Most people would probably say yes. Are there different approaches that are effectively used by various coaches and athletes? Yes.
For those interested in the particulars (especially pulling styles) of the quick lifts and the power lifts, let me suggest you peruse Dr. John Garhammer's web page of selected publications. In particular, his chapter five in Biomechanics of Sport is quite informative. You can download it in PDF form here.
Garhammer, J. "Weight Lifting & Training" (Chapter 5, pp.169-211). In: Biomechanics of Sport (C.L. Vaughan, ed.), CRC Publishers, Inc., Boca Raton, FL., 1989.I think most people will find Dr. Garhammer's discussion of the mechanics of elite deadlifters and elite weightlifters quite interesting.
Personally, I think it is quite reasonable to think that a clean/snatch requires a different pulling style from a deadlift. They are very different movements with very different requirements with regard to bar height, velocity, acceleration and power output. It's like comparing hurdles vs high jump.
The weightlifter interacts with the barbell in a much more complex way than the powerlifter, moving the barbell a greater distance over a shorter amount of time. Weightlifters have to actually move their bodies around the barbell, while powerlifters do not. The complexity of the movement is not only reflected in the bar trajectory, it is also reflected in CoP (center of pressure) measurements of the foot during snatches and cleans.
While it would nice to be 100% efficient and have a purely vertical bar path, humans are humans and not pulleys. Humans are not world-beaters in mechanical efficiency of any activity. Thus, we see horizontal movement in bar path analysis of elite weightlifters. Most researchers have shown there are roughly 3-8 cm of horizontal displacement in the barbell trajectories of successful lifters. And there are even categories of bar trajectories. Tommy Kono has a very nice discussion of the various S-shaped barbell paths in his book.
We also see that most weightlifting athletes start with the barbell over the metatarsals, so the barbell can sweep in toward the body as it passes the knees to the mid-thigh (clean) or upper thigh/crotch level (snatch). While we want to keep the barbell close throughout the lift, it appears to be most effective for the weightlifter to start with the bar slightly in front of the lifter, rather than start with it close like the powerlifter.
Many times, if the lifter starts with the barbell too close the body (crowds the bar), the barbell has nowhere to go but forward, and the athlete ends up having to jump forward (see below). If the athlete is going to jump anywhere, backward is usually more reliable than forward. The athlete on the L missed; the athlete on the R did not. Most weightlifting coaches would say the athlete on the L started with the bar too close.
This starting position (bar over the metatarsals) also allows the weightlifter to use their knee and hip extensors effectively to push the weight off the platform during the first pull vs. pull the barbell off with a more hip/back dominant strategy.
So that's my take on the subject. Read the Garhammer chapter and some of his other published work, and see what you think. Is it reasonable to think the weightlifting and powerlifting pulling styles might be different, given the demands of the lifts and given the analyses of past and current elite lifters? Is there good reason for weightlifting coaches to teach pulling the way their do?
For those of you who haven't seen V-scope data of lifts before, below is the data from the best snatch and clean by Natalie Woolfolk (94 kg and Am Record 118 kg @ 63 kg bw) and Kendrick Farris (155 kg and 190 kg @ 85 kg bw) from the 2006 National Championships. Although you cannot see where the barbell starts in relation to the foot, you can see the horizontal movement in the bar path that is typical of elite lifters, and these are two of the best in this country.