Anchor or guylines are required to maintain stability on yarders. Here is an example of a grapple equipped, swing yarder landing logs in the chute. The cable setup shown here is a running skyline.

The guyline tailholds must be changed as the yarder moves and kept in line with the yarding lines.Swing yarders such as this require only 2 guy lines and these are in proper lead to the skyline position.