Hi geobruce, and welcome! I don't know of a FreeCAD tool does what is shown in the video. It would probably be a nice tool to request in the bug tracker.
You can use the Part WB > Cross Section tool:
but the cross-sections it creates are 2D sections and don't update if they are moved, and the result is probably not what you want.
You could also position a large Cube over a portion of the shapes:
then use a Boolean Difference to subtract the Cube from those Shapes, resulting in what we call "Cuts":
Moving the Cube will cause the Cuts to update, but they do not update in real time.
You could probably do something with a Part Design WB > Datum Plane but I suspect the result is also not what you want (I simply haven't tried that yet).
OS: Ubuntu 16.04.4 LTS
Word size of OS: 32-bit
Word size of FreeCAD: 32-bit
Version: 0.17.13522 (Git)
Build type: None
Python version: 2.7.12
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: English/UnitedStates (en_US)