The Stein RC3002 is the latest in a range of rigging devices designed by Reg Coates and marketed by Stein (the number in the name refers to its working load - 3000 kg. The 2 refers to the number of bollards).The lever at the top is for tensioning the load (not really for lifting although theoreticallyyou could lift some weight if you used a block up top). Looks heavy but weighs less than 12 kgs.
Apparently the rope grip is quite unique and not at all like an ascender. It is made up of a series of cylinders slightly offset from each other thatclamp together onto the rope, creating more grip the harder you pull the tensioning bar.
According to Reg the rope grip does not damage the rope any more than a knot does under load. The grip has an adjustment screw to ensure you don't pinchthe rope too tight. This can be adjusted to accommodate various diameter bull ropes.
The dual bollards allow for ropes to be attached to both ends of the branch being lowered. Follow the Treebuzz thread hereifyou want more information and plenty more pics (at the bottom of the thread).