Internet download manager claims hat it boost downloading speed up to 5 times.
But how?

I have downloaded some software with it, max speed remains the same as my data plan is

 

which brings us to the 'one born every minute' or the 'stanley meyer chronicles'

actual download speed won't change unless you pay for a faster service from your provider.

 

It depends what you measure against. Download managers often use things like compression to compress the data as it is sent, thus reducing the volume of data and making the transfer faster. However a lot of the files you download are already compressed so there is little or no advantage. But to get the best marketing figure they can compare downloading an uncompressed ascii file with lots of whitespace and then claim 5x the speed.