José Wallois who was using this device explained to me its principle in 1993:
Each end of line "a" is tied to the holding-bar. Both ends go through a ring "b" to another ring "c" where the line slides. This ring is fastened to a rubber band "d".
Depending on the elasticity of the band, the swing of the pendulum is more or less slackened. It is light, adjustable, and efficient. I use it systematically.
  For more efficiency, a titan rod is extending the holding-bar at its upper part..
I thought that Michaël HAUGRUND was an historic man of the beginning of the XXth century. It was only when consulting the list of AERIAL EYE subscribers that I saw he is an historic man still alive! He had shown his device to our German kaper friends, and had shared it with everybody in the KAPWA bulletin.
Danke schönn, Michaël !