Complex sentence: The sprinkler pivots on a bearing on top of its threaded attachment nut, and is driven in a circular motion by a spring-loaded arm that is pushed back by the water stream, then returning to "impact" the stream.
Split into simpler sentences: The sprinkler pivots on a bearing on top of its threaded attachment nut. It is driven in a circular motion by a spring-loaded arm pushed back by the water stream which returns to "impact" the stream.