I have used the channel under the bathroom floor. I think I pulled all the existing wires taut, pushed them to one side as well as I could, then straightened a wire coat hanger, bent a small loop into one end, and poked it through. When the loop came through, I tied the middle of a long piece of Venetian blind cord through it, and pulled the coat hanger back. This left me with two lengths of Venetian blind cord through the channel. I tied one of them off on both ends, and used the other for pulling the new electrical wires through. Just as a note, the new wire was fairly stiff (Romex, maybe?), and I thought I would be able to simply push it through. Nope. It snagged every time, and I had to pull it.
The channel under the bathroom floor is simply chopped out of the foam that makes up the middle of the floor's structural sandwich. If needed, you can chop out a bit more foam to make more room in the channel. Handy. You may not be able to do this with the channels in the ceiling if there is trim that needs to snap back into place. But I have not tried it.
Bill
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