It’s inserting short, hex head screws into the lugs in the outsole (ie, the bottom) of the shoe For an excellent write up on how to do this, read Matt Carpenter's webpage, "The Screwed Shoe".
50K, 20 Mi & 10 Mi Night Trail Races
Hawk aid station at 2017 Heartland 50
Trail Hawks Ridgeline aid station at Heartland100, 2018