My understanding from the 626 I have, and the discussions about this that I have seen elsewhere is that the 6x6 anti-tips are spring loaded, and may have some adjustment in the height of the casters off the ground.
I think I've seen notes of people swapping out the wheels for smaller roller-blade wheels.
The amount of ground contact seems to have increased over the years - reportedly the newer chairs have them in contact all the time, while the older chairs had them off the ground.
They are spring loaded, and in some cases, particularly chairs with tilt/recline or other COG shifting stuff, may have mechanical stuff to either lock them or stiffen the springs when the COG shifts backwards.
I have seen reports of people putting blocks or other stuff into the mechanism to pre-load the springs and get the wheels off the ground.
Jazzy Select 6
Want to make / get a better chair, ideally one that stands.