upper part that runs as a long strip down the back of the inside of the shaft
Heel slides reinforce the upper against crushing and sagging, provide a smooth surface for the heel to ride along when putting shoes on, and provide additional thickness against abrasion from the heel, preventing or delaying the development of holes in the heel lining.
Heel slides are particularly common in tall-shafted pull-on boots, such as a cowboy boots, where they also hep to reinforce tall boot tops.
Compare backstays on the outsides of uppers.