This is the three element Yagi I'll be trying out in this week! The guy wires are suspended from trees at each corner. The first set of 'elevation pulleys' allows the ends of the parasitic elements to be raised and lowered independently so that they can be leveled.
The second set of 'tension' pulleys allow the parasitic elements to be removed altogether by lowering them to the ground so that various metrics of the antenna, (gain, impedance, etc...), can be measured with and without them.
Each pulley set is suspended from the guy wires in such a manner that the element can be slid along the guy wire to adjust element spacing.
Plastic and glass string levels are attached to each of the elements and on the cross element level string. The levels are used to make adjustments to ensure that all the elements of the antenna lie in the same plane. The levels are read with a pair of binoculars.