Peaking at words you already know should diminish that word's strength.
I've been noticing recently that I'm coming upon a lot of words that I'm not very comfortable with, but still display as having high strength. I may be mistaken, but it seems as though peaking at a word does not affect that word's strength. Changing this might make people more inclined to go back an practice previous lessons more often.
My latest understanding of Peeking is that the system will introduce the word that was "peeked" into your lessons more often so you can master it. If my understanding of the decay process it correct, this response to peeking would actually increase the strength of that word because the system is theoretically introducing it into your lessons more often.
In fact, sometimes I copy for myself full sentences whose structure is surprising for me, and while doing so, I accidentally hover over some of their words, which qualifies as peeking, even though I didn't have that intention. So maybe there should be an easier way to copy sentences (and their correct translations) automatically without having to bother with the mouse pointer over them, thus avoiding unintentional peeking (and misleading the system).