In the above reaction product '$$P$$' is :

The SN2 Substitution Pathway

In a substitution scenario, the cyanide ion (CN-) acts as a nucleophile rather than a base.

• Attack: The CN- ion attacks the carbon bearing the bromine from the backside.
• Departure: The Bromine atom leaves as a bromide ion Br-.
• Result: The -CN group replaces the -Br group, leading to the structure  

It is important to note that if the question mention alc KOH then we would be using and elimination pathway, however as the question mentions KCN we must understand that KCN is a substituting agent than a solvent.