The frame rate of the camera coincided with the speed of the helicopter blades

This effect has been described more than once, but it still looks impressive

The frame rate (usually measured in fps, frames per second) shows how many pictures are changed per second of video recording. In the case of shooting rotating objects - for example, helicopter blades - during the time that passes from shooting one frame to shooting another, the blades have time to travel some distance. If the frame rate coincides with the frequency of rotation of the blade through 360° or with the time it takes for the blade to take the position of one of the neighboring blades, then the camera captures the same pattern every time, and it seems that the blades are standing still. And if the shutter speed is short, then the blades will be very clear - just like in this video.