I honestly expect we see teams man the mark 5m back and dare the teams to play on into that space rather than be up on the mark and absolutely handcuffed from any lateral movement. Basically you just start things 5m deeper and man it how you normally would and should actually be able to circle closer on the corridor side as a result, teams will allow the opponent the boundary as they always do, perhaps even more pronounced now.
Also think we see a lot of the standing over players on the mark to slow it up which given players aren't allowed to move from the mark will have to be allowed more by the umpires, can't tell them they're not allowed to move an inch and then expect them to move.
Going to be an incredibly important rule though as depending what teams do it's going to have a drastic impact on SC scoring.
Take for example, stand 5m back, guard corridor religiously. This should favour intercept marks and ruckmen as you'd expect a lot more long kicks to the boundary.
Stand up on the mark but flood extra numbers to cover for players being worthless in the zone while manning the mark and it's the seagulls likely to get a lot of cheap marks as teams maintain possession and work to try and create openings and expose that player that can't do anything until they get an angle on him.
If it leads to much faster ball movement then leading marks become a lot more likely and run and carry players should shine.
This is going to be one of those seasons where the r3 correction trades could be exceptionally important, maybe to the point that it's worth using an extra two in r2 or r4 or even both. Think this could be like the kick-in rule on steroids which basically added 10+ points to everyone taking them and fundamentally shifted the back position to create a new tier for them instead of them generally being equal with forwards, the hard part is right now having no feel on how it work and not expecting that teams will go with their A plan in the preseason.
To me it still makes the most sense to man up 5m from the mark, which would still allow the player to play on a bit quicker as they wouldn't need to get back off it often but would mostly undercut the change, allow corridor guarding and if the player does play on ahead of the mark you can come up and close the space while also taking the 50m penalty out of the equation for the most part. This to me would have the least impact on SC scoring if coaches go down the path but given it's the logical and common sense approach I'm not sure if any coaches will actually think of it.