Although it doesn't show in the pic, the flex puller has a "cap" across the angled end. You can see the hole on the bottom for attaching it to the fishtape.
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Yes it's a fishing sinker. At 4 pounds, it's 1-1/4"X 1-1/4"X 5". Pulls right out of a hole for a cut-in box.
There's a 20' length of #10 Jack Chain attached. You can use one end of the jack chain in hollow walls or small holes. Insulated walls don't offer much fight when it's turned around sinker end first.

Guys laugh when they first see it, but then often ask to borrow it after seeing it work. [Linked Image]