transformer fuse cutout
Briefly they are a combination switch and fuse to protect transformers and to protect overhead lines from damage. As a switch they can be opened by a lineman using a hotstick and as a fuse they will automatically open when a fuse melts if the current gets too large.
To fully explain how they actually work would take me a good deal of time and writing but there are excellent answers already out there.
Just typing your question "how does a pole mounted fuse cutout work" directly in a search engine like Google will provide a wealth of information including specifications for different cutouts.
Of particular interest might be the Wiki entry on it and even better are the many many Utube videos of not only how they operate but showing you how and showing you how to use them if you were a lineman for an electric company.
p.s. "Let's Talk About Power Lines - Reclosers vs Fused Cutouts ..." got me to the uTube videos.