Using technically impossible shutter speed when shooting video I have a question about the video mode of my DSLR.

When i record i can set the frame rage (e.g. 25 fps). In that setting my DSLR should take 25 images within 1 second.

In manual (or "s" mode) mode i can set the shutter speed of the taken video. 
Technically the  maximum should be 1/25 because of the frame rate. But i can also choose values above. 

Question: How is it technically possible to record a video with 25 fps having set the shutter speed more than 1/25s. ( e.g. 1/4s)
Whether this happens inside your specific camera is not clear. However, it is possible to have a shutter speed slower than the FPS would naively allow. In practice, it's very unlikely that a camera would do this if uses a mechanical shutter for video, but it can be practical in electronic shutter cameras.

Imagine a shutter speed of 0.5 seconds, and a framerate of 4fps. Each 'exposure' lasts 0.5 seconds, but each frame's exposure overlaps with neighbouring frames - the frames are not the same, but they overlap. 

Imagine:


A is the light value that arrives between 0 and 0.25 s
B is the light value that arrives between 0.25s and 0.5s
C is the light value that arrives between 0.5s and 0.75s
D is the light value that arrives between 0.75s and 1s
etc...


Then:


Frame 1 contains the light A+B
Frame 2 contains the light B+C
Frame 3 contains the light C+D
etc....