Curious as to why you believe "free spinning" is better
The idea is to WORK ... and some resistance builds more work than none.
When we run naturally, our legs don't just run "freely"; they propel us (or bodyweight) up and forward.
When we run uphill (or on stairs), etc., the resistance is thereby increased, and the work is thereby maximized.
These same truths apply to mills and dogs.
A completely "free-spinning" mill is therefore the LEAST difficult (therefore the least effective) mill at conditioning a dog for a physical fight IMO.
Now, you don't want too much resistance either, to where they can't turn the mill hardly at all, but some resistance is definitely preferable to none.
Jack
Jack