My Vari-X III has this feature, and it works only with the dual-x reticle. IIRC, what you do is bracket a 9" object between the horizontal reticle and the top of the thick part of the bottom post (or an 18" object between the thick posts on the vertical reticle) by changing the power until it is bracketed. Then, you can read the distance in hundreds of yards off of the power ring. The distance markers are on the opposite side of the power ring from the power markings.
I really like this feature, as I can also use the top of the thicker bottom post as a quick holdover on my 300 WM for a 300 yd shot with the 9" distance at the proper power. Faster than turning turrets...
Hope this helps,
Brian