By transmission mechanism I mean something more general. X has a non-negligible effect on Y, Y on Z etc.

An example of implausible transmission mechanism:

• This or that food contains antioxidants, antioxidants protect from molecular damage, less molecular damage means you'll live longer - it has 2 totally broken links since entirely insignificant amount of antioxidants from food get anywhere in the body, and putting more antioxidants in the cells doesn't actually do much at all (even if reducing their amount increases damage considerably).

An example of plausible transmission mechanism:

• Having dogs improve people's moods, people in better mood have lower blood pressure, lower blood pressure decreases risk of major cardiovascular disease - we don't have much hard data here (funnily enough they did a randomized study once, and found such effects), but every link in the chain is plausible and effect is within realistic order of magnitude.

With sufficiently overwhelming evidence it might be reasonable to ignore lack of any plausible transmission mechanism, but evidence is anything but, and I'm more inclined to think that it went from "I need to publish X papers a year" to "finshing for statistical correlations involving lithium" to "publishing a paper about that".