How this all works dates back to the original design of the Magic system.I wanted Ruin and Preservation to be complimentary opposites, like many things in the Mistborn world. Allomancy, for example, has Pushes and Pulls were are less "negate one another" opposites, but instead two sides to the same proverbial coin.
Ruin is invasive. The power is more "Yell" than "Listen." The philosopher would probably have some interesting things to say about the masculine symbolism of Hemalurgy and its spikes.
Ruin can insert thoughts. That power, however, can't HEAR the reactions. It's about invasion.
Preservation, however, is the opposite. Preservation listens, Preservation protects. (Perhaps to a fault--if there were no Ruin, there would be no change to the world, and life could not exist.) Because of this, Preservation can hear what is inside people's minds. It cannot, however, INSERT thoughts. (This is important to the plot of Hero of Ages.)
Harmony is both, the two complimentary opposites combined. And so, he inserts thoughts with Ruin and still uses Hemalurgy. He can also listen.
You can see it all here: http://www.reddit.co...d_alloy_of_law/
(Also, can we get all of the random questions that have been answered through reddit on the database? Please?)