When opposing weather fronts meet - warm and wet, cool and dry - the heavens thunder and crackle with raw electrical force. At shrines throughout Japan shide (紙垂), folded paper streamers hanging from shimenawa (注連縄), the braided rope that surrounds sacred trees, torii gates, and other objects of veneration appear to the distant observer as stylized lighting bolts.
O-shide mark the boundary between the spirit world and the floating world. Just like the signs on fences and gates of electrical works - beware of crossing from this world to the other.
Kodama (木霊) sprits inhabit certain sacred trees at Shinto shrines. You will know which ones by the presence of the o-shide hanging from the thick rope wrapped around them. To harm the tree is to harm the kodama, which will surely bring bad luck.
O-shide are not, however, limited to trees and Shinto shrines. They are also used to decorate anything considered sacred. You can see them at Kabuki theaters, on matsuri mikoshi, decorating the belts of sumo wrestlers.
I watched them being tousled in the wind on a blustery day on the last day of the year. They were hanging around a huge old pine housing an ancient kodama. I kept a safe distance while admiring the tree.