Please can someone explain to me how to use Internet Download Manager (IDM), how it works and also where/how I can download the crack version for free on my PC.
Your suggestions would be highly appreciated.
Thank you.

What actually happens is -->
Most of the servers allow more than one connections to a SINGLE FILE STREAM [ any file consists of stream of bits ] from any client IP address requesting the file.
IDM first checks whether multiple connections to the file can be made or not. If yes, it can download in parts [ Explanation below as how it is actually done. ] otherwise it downloads the file linearly and at the same time the download can't be paused.

Now that IDM knows that the server allows multiple connections. It does this.
An application [here IDM] makes requests to the server for the file that it wants to download and the server returns a "BYTE STREAM" to the application

Now the trick that IDM does is that it makes 8 requests to the server at the same time [ using Threads ] and in all the 8 requests it sends different starting byte addresses [ which will be the starting bytes in the Byte Streams that will be returned] so that all of the file is covered.

For example if the file is say 8000 bytes large, what IDM will do is,
It creates 8 requests to the same file on the server simultaneously but with initial starting bytes as..
In Request 1, starting byte = 0. Bytes covered: 0-999
In Request 2, starting byte = 1000. Bytes covered: 1000-1999
In Request 3, starting byte = 2000. Bytes covered: 2000-2999
In Request 4, starting byte = 3000. Bytes covered: 3000-3999
In Request 5, starting byte = 4000. Bytes covered: 4000-4999
In Request 6, starting byte = 5000. Bytes covered: 5000-5999
In Request 7, starting byte = 6000. Bytes covered: 6000-6999
In Request 8, starting byte = 7000. Bytes covered: 7000-7999

Now the returning byte streams will start from the respective bytes in the actual file. The parts downloaded from each of the requests are then joined together in order to make the complete original file, after the downloading of every stream completes.

Does IDM increase my speeds or downloads more than my connection can, in given time?
No IT DOES NOT. Most of the users, even many experienced ones, have this misconception.
What it actually does is, that it just optimises the way your connection is used i.e. it ensures your connection is fully used by the different downloading threads that it has created. It ensures your connection is never idle so that you can get the maximum possible speed. THATS IT.
It just provides a method to manage downloads efficiently and make the maximum use of your connection that can be possibly made. And it does so brilliantly.
In my view, it is one of the best applications ever made for Windows in modern times.