With fgrab, capture any action on your screen and save it as a screen movie. In addition to the video capture one can record the Mac system audio and a voice-over. After the recording the screen movie can be saved as .fvf (fgrab video format). Double-clicking this file brings up fvf_player which allows you to set in and out points (among others) to prepare the insertion to FastCut movie projects. .fvf is a custom format made for a special purpose that is screen capturing. Use FastCut (the advanced video editor from the same developer) for further video processing.
Under the hood, fgrab has a highly sophisticated capture engine. fgrab only captures the area of the screen that is in motion. CPU load and the amount of data written to hard disk depends on the screen action. No screen action implies no CPU load / no data written. fgrab is way faster than most other tools on the market (learn more in the enclosed user's guide!). Hence creating fgrab necessitated great programming efforts.
Note: fgrab was not ready for the latest Mac operating system since the functionality of some OpenGL APIs had altered. But fgrab was not mothballed; the super-fast utility has been partly rewritten and revived.