there is an great explanation on quora but I can't find it anymore
basically, it means that IDM downloads file parts and store them randomly on the disk. After the download is finished, IDM joins those parts and causes extremely high disk usage (only noticeable with large files), similarly to defragging your SSD/HDD -> bad for your SSD
with other downloaders, for example, the good old Eagleget, the file is pre-allocated on the disk and file parts are downloaded to that specific allocated spaces. Therefore, after the download is done, there is no need to join or the joining process can't not be felt because nothing is moving
I just downloaded a 12GB file with xtreme download manager. My laptop was unusable during the file joining/assembling. The SSD usage was 100% -> uninstalled
