We install them when we construct metal fencing.
Removable rail sections? expansion?
To accommodate seismic forces?
Yep, expansion control.
When someone grabs on to hop the fence the bar spins and they fall flat on their face to deter them from trying again ??
I like Tony’s idea, might help the keep thieves out of your yard. However, since it’s at one side only in that section, I’d say an expansion joint as well.
Another thought. They are prefabricated in a shop to make for quicker install onsite.
They are expansion/contraction sleeves. A 200’ metal fence expands/contracts 5" due to temperature changes.