Punctuation query

I hope that this rewording will help:

Surfing was originally developed by ancient Hawaiians. It appeals to people for several reasons:
1. the sport’s unusual confluence of adrenaline, skill, and high paced manoeuvring;
2. an unpredictable backdrop that is sometimes graceful and serene, and at other times is violent and formidable; and
3. the camaraderie that often develops among people in their common quest to conquer nature.

The expression 'by turns' means that the backdrop alternates between being, on the one hand, graceful and serene, and, on the other hand, violent and formidable.
The two extremes each take it in turn to predominate.