Echo PRO ATTACHMENT Series Operator's Manual page 17

 Do not push the line into tough weeds, trees, or wire fences.
 Pushing the line into chicken wire, chain link fencing or thick
brush can result in snapped-off line ends being hurled back at
the operator. The proper way is to cut right up to a barrier, such
as any of those mentioned, but never run the line into or
through the obstruction. Do not cut closely to obstruction or
barrier.
 Avoid nylon line contact with broken wire fencing. Pieces of
wire broken off by the trimmer can be hurled at high speeds.
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Operation