The key is you cant "initiate" "forcible" contact with your shoulder, forearm, helmet, etc.
You can get in his way, you can ride him, you can redirect him you can go down in a tangle of legs with him, but if you pop a guy coming across the field you are going to get flagged.
The reason the rule now exists is that it's a treacherous area as far as taking concussions (whether its the actual contact, hitting the turf or even the whiplash)