The gin pole redirects the force on the line being used to raise the mast.
Without the pole the line would tend to lay along the mast and tension on the line is mostly attempting to make the mast shorter. Essentially zero force is working to lift
The gin pole changes it to a triangle of forces which will lift
the "top" of the mast and pushing down at the step.
As the mast goes up the gin pole leaves the force triangle, (in most cases... not the one in the video) with the line used to raise the mast rising off the pole. As the mast becomes more vertical it requires less tension on the line to raise it further due to the geometry of the forces.
Actually, this is all applying High School
Geometry and algebra to real life...
I use bungees to have light tension on the side stays which does a wonderful job of keeping the mast from waving from side to side as it goes up.
You need to ensure you have blocks of some sort to prevent this from kinking the stranded stainless wire stays.