How internet download manager increases the downloading speed?

Answer 1

Usually, if you're downloading a file, the download server is limiting the download speed, and not your internet connection (though this might be different if your connection is slow)

The download manager splits the file into multiple segments, and downloads the segments from different servers, so that every segment has it's own download speed for a total download speed that is far higher.

Answer 2

The main purpose for an internet download manager (IDM) is to increase speeds for the user when downloading. IDM's accomplish this by dividing downloads into multiple streams.