Its definition:  An original (or fixed) sukoon is positioned after a medd letter, in a word or a letter.
By original it is meant that the sukoon is part of the original make up of the word, and is present when continuing the reading and when stopping.

Its Divisions: is divided into two groups:

      1.  The Compulsory Word Lengthening

      2.  The Compulsory Letter Lengthening

Each of these two divisions is further divided into two more divisions. We therefore, have four divisions of the .

      1.  The Compulsory Heavy Lengthening in a Word

      2.  The Compulsory Light Lengthening in a Word

      3.  The Compulsory Heavy Lengthening in a Letter

      4.  The Compulsory Light Lengthening in a Letter

We will explain the first two (The Compulsory Heavy Word Lengthening and The Compulsory Light Word Lengthening) this tidbit lesson, and the next two, insha’ Allah in the next lesson

The Compulsory Heavy Lengthening in a Word

Its definition: It is when an original sukoon (the letter has a shaddah on it ) comes after a medd letter in a word. 
The word heavy  refers to the shaddah.  A shaddah indicates two letters of the same, the first one with a sukoon, and the second with the vowel that is accompanying the shaddah. The two letters have merged 
() into each other, and thence the shaddah. It is called  due the permanent, or original sukoon found when the reader stops or continues, or because all readers agree that this lengthening must be 6 vowel counts.  It is called  due to the fact that the medd letter is in one word. 

Examples:

Click here to hear this lengthening

The Compulsory Light Lengthening in a Word

Its Definition: It occurs when an original sukoon that is not merged (no shaddah on it), follows a medd letter in a word.  The word light ( ) comes from the letter not being merged.  This refers to the letter with the sukoon that follows the medd letter.

Places of this lengthening:
There are only two places of occurrence in

one word of this kind of lengthening in the Qur’an.