"Real" Ducking / Crouching allowed or not?

[Haco]

Hi, quick question to make it clear:

Is ducking/crouching whatever you call it allowed  -  as long as NOT using the mousewheel?
To make it clear - Iam talking about this:

https://youtu.be/AFXo4U35_5k?t=53s

Please watch before you answer.

Thx