Clean technique

Hey guys just wanted to ask any of you experienced in Olympic weightlifting your thoughts on wether or not one should bump the bar when performing clean and jerks, power cleans or hang cleans.
I've always focused on high pulling the bar during the brush/sweep rather than bumping with the hips as this was the technique taught to me by several coaches over the years - including 2 Olympic level coaches.
But as I see more people performing Olympic lifts these days I also see what appears to be a hip bump.
Either my eyes a tricking me into seeing a bump due to the massive hip drive or these lifters are hitting the bar with the hips during the brush.
This has been a bit of a grey area for me over the years and would love to get a much more black and white picture from a coaching perspective.
Thoughts please.

It's not really a 'bump', it's more of a 'touch' as your hips drive forward and extend as you high pull the bar.

Squatdog - It's not really a 'bump', it's more of a 'touch' as your hips drive forward and extend as you high pull the bar.
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I've seen it not mentioned.  I've seen it mentioned as a "touch" like Squatdog said.  I've also seen it mentioned as a full on bump trying to take advantage of bar flex in propelling it upwards.  All were from experienced coaches and/or national level lifters that I've had the priviledge of training with.  I'd imagine they are all right.  It's a complicated, technical lift, but there is room for personal preference.  If a bump doesn't work for you, then drop it.  

This is my experience:

IMO a clean isn't really a clean without the hip drive. The pulling phase of the clean is simulating the full extension of the body during a vertical jump, which you are adding resistance to. Without the "bump", you are mostly using your arms and back to clean the weight off the floor, whereas using the hips is more functional for your legs and can generate more power.

You will probably notice a big difference at higher weights.

Just to add to my point:

I see regular guys in the gym high-pulling the bar all of the time, but I have never once seen a professional lifter, in competition or otherwise, do a clean without the hip drive through the bar.

Like I said, it's more of a touch than a 'bump'.

If you're bumping it hard with your hips, you aren't finishing your top pull.

Cheers guys. Some good advice.