It does work.
You have to remember that PSUs are designed to have power come OUT of the positive voltage supply lines, then back to ground.
When you do the 7 volt trick, the power comes out of the +12 volt line, but then you're expecting it to come *into* the +5 volt line, which the PSU just wasn't designed to do. However... If you have something connected to the +5 volt line, then the power can keep going through whatever you have connected there (at +5), and return to ground.
Without any load connected between +5 and ground, the PSU's internal protection circuitry will shut down the PSU (in most cases).
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