Tunnelling is a difficult issue to be dealt with in footy because its a very different game to basketball.
In basketball you have a protected vertical cylinder around you so you can jump up and not have anyone above or below you. This way you can hold a position in a non contact sport. You can also have a space reserved for your landing. raher than a collision when you are likely to be off balance and vulnerable.
In footy players tend to run at the ball, make contact, cut each other off and then jump over each other. Often one jumps early, into and over the other to be helped up by the second players spring to the ball. In this case the jumping over effectively takes out the lower player who can only respond by removing his support, and move away before recontesting for the ball.
Also the player jumping over the front player can put a straight leg and foot into his back or a bent knee, and both can cause damage to the front players, skin(stops), back, kidneys etc.
So the player in front is a target for boots, knees and jumping over, and if he shifts position in retaliation then its called tunnelling. All difficult decisions for an umpire to adjudicate.
Specifically the still photos of Bolton and Riewoldt in the Age showed Riewoldt jumping quite early and over Bolton, so he was interfering with Bolton's line and elevation going to the ball. Bolton shifted to take his support away from Riewoldt, and probably then pushed underneath him too.
So the issue of jumping too early to take a player out of the contest needs to be balanced against the deliberate taking out of the legs, hips of the upper player.
I think the key issue is not moving off the line towards the ball by either player. Therefore both players would move straight from their starting position towards the ball, and not interfere with each other other than jumping for the ball or if their lines intersected before the ball arrived, and then issues of shepparding, in the back etc could be judged.
Based on this adjustment to the rules, contact in the back or side by a foot or front of a knee should be a free kick, contact by a glancing shin, side of the leg, hip etc in a marking contest would be permisible except in an unreasonably forceful contact.
So encourage players to fly for the ball, without hands in the back, or without deliberatelyKneeing or forcefully taking out the other players also flying for the ball.
Similar to banning head high tackles and bumps, encourage the skills, but lessen the dangerous physical impacts.
Make it a safe (work) place for the contestants because there are enough accidental injuries and collisions anyway.
Signed a not a soccer mum, but someone who wants a tough fair courageous football game, without intentional violence or thuggish intent.
Next post will be my thoughts on the tackling/holding the ball rule change.