Increasing the flanking DNA from 40 to 80 base pairs (bp) causes ~100-fold faster nucleosome sliding by INO80. A prevalent hypothesis posits that the Arp8 module within INO80 enables a ruler-like activity. Using cryogenic electron microscopy, we show that on nucleosomes with 40 bp of flanking DNA, the Arp8 module rotates 180° away from the DNA. Deleting the Arp8 module enables rapid sliding irrespective of flanking DNA length. Thus, rather than enabling a ruler-like activity, the Arp8 module acts as a brake on INO80 remodeling when flanking DNA is short. This autoinhibition-based mechanism has broad implications for understanding how primitive nucleosome mobilization enzymes may have evolved into sophisticated remodelers.