Putting your head down and using the top of your helmet is not being a good tackler. Keeping your head up and using your shoulder and body to bring the guy down is clearly the better way to go.
You never see a guy get one of these helmet to helmet hits when he's running in the open field with one man to beat. Why? The defender has to make sure to keep the man in front of him. That means keeping your head up and making a good tackle.
The main times you see these stupid, pointless hits are on plays where the player being hit is:
1. Looking to catch a ball/just recently caught a ball
2. Already being tackled by someone else
3. Running and looking in another direction (blocking, such as in the video that began this discussion)
Those are times when you can get away with being an idiot and taking your eyes off a guy and using your helmet to smash into his brain.
The "if you leave your feet" thing is a stretch, the NFL has it worded well when it says "leading with the helmet" and "helmet to helmet contact" are the issue here. If you leave your feet and drive your shoulder into a guy, he might get the wind knocked out of him or break a rib (or hurt a joint if it gets stuck in the ground or on another player, but the direct impact of the hit only causes broken ribs or simply knocking the wind out), but you could be on the ground and drop your head and break a guys neck with your helmet. I think we can all agree which is preferable.
If you enjoy seeing concussions and paralyzations, I'm sure you can talk to WWE about starting another football league. Tell them you'll pay 99.95 on pay-per-view to watch James Harrison wanna-bes running around throwing their heads into each other until only one man leaves the field.

Which, btw, his (Harrison) "I think I'll retire if I can't play the way I always played" cry-baby act was one of the stupidest things I've ever heard in sports.
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