To dump video, you need the following: DeckLink SDI card, with signal input to it.

Star the Blackmagic Media Express to make sure that the input signal is available.

Select the proper device from the Device menu on the top.

Go to Log And Capture tab, if the video is available, you can press the Capture button.

Pressing it again will stop the capture.

By default recordings will go to Documents folder.

This is how it looks when it's recording.