Yes. They are easy when you know how and impossible when you don't!
This is an ultra nifty Chinese design.
Insert a small screwdriver against the shiny metal slot below the slot where the wire goes in. Voila! It opens! Stuff your wire in and pull the screw driver out and its locked tight.
Its really quite amazing.
It took me ages to find how to do it but theres a Youtube video...
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Notes on a Circumnavigation. OurLifeAtSea.com