The Thompson wheel self-limits depth by virtue of the widening of the spoke from the tip to the base, so that the spoke encounters considerably greater resistance as it gets closer to penetrating the full length of the spoke.  (And the spokes aren't overly long).  Looking at the spokes from the side-view, they are much like a triangle with a blunted tip.  Here's a photo, and description of various spoke types on the market:

http://www.exapta.com/products/thompson.html

(scroll down a bit)

The Thompson wheel has considerably less tendency to go too deeply as compared to 'spike' type closing wheels with very long slender spokes (i.e., the ones that are old-school straight-tooth row cleaners), yet part of the depth-limiting on the Thompson wheels is still accomplished by running sufficiently light spring pressure on the closing bracket. 

For all spoked or tined closing wheels (except those with depth bands), the amount of toe-out strongly affects penetration depth.  This includes whether the planter is running nose-down or not, and whether or not the spoked/tined wheels have altered the angle of the closing bracket at all (due to wheel diameter).   Also, how closely the spoked wheels are spaced to the row affects the amount of action / penetration.

I hope this helps,