When hooking up a floating tripwire trap, I noticed the string would not connect to the tripwire hooks when there was a block placed under the west trip wire hook. When the entire system was above air, it works, contrary to minecraftwiki.
I then set up 4 directional trip wire hooks with a block under a north, south, east and west facing hook to test which worked. Enclosed is an image below of a compacted version of my experiments.
When a block was placed under the south or west tripwire hook, the string would not connect and a redstone pulse will not be received.