Unlike standard downloaders that segment files before starting, IDM 6.21 segments files dynamically during the download process, which can increase speeds by up to 5 times.
During its lifecycle, several "builds" of 6.21 were released to fix specific bugs: Internet Download Manager - 6.21 Build 2 - Filepuma.com idm 621
While modern versions of IDM have advanced further, version 6.21 established several core technologies that users still rely on today: idm 621