The “Video Tachometer” app for the iPhone was used.
App description: https://usefulmobileapps.com/en/video-tachometer.php
App Store: https://apps.apple.com/app/id1492583587
The video shows the BELL 429 helicopter. The rotor speed of the helicopter must be known in advance. Because the helicopter arrives/departs very quickly. Typically, the larger the helicopter, the lower the main rotor speed. The direction of rotation of the main rotor of the helicopter may differ even for helicopters of the same company.