I thought I'd share a tidbit about how I study, specifically with Duolingo.
Whenever I open the Duolingo app on my iPhone, or in the web, the first thing I look for is weakened words. Before I even consider starting a new section, I always strengthen those words first, and I do ALL of them until the gauge fills up.
I enjoy strengthening my words in the web version of Duolingo, because I love the timed mode (it's been a while, but I believe you have to spend Lingots to unlock timed mode*.) I like it because it makes me think on my feet, and the challenge feels more like a real conversation, where you might only have a few seconds to respond. It causes me to simply think faster.
( Also, it's such a great feeling to see all the gold medals :D ) * I really hope timed mode comes to mobile.
In timed mode if I instantly think of the word, I know that I've got it down. Whereas if I can't remember it under the pressure, I add it to my flashcards to study later on.
Even better, with the web version of Duolingo, after a lesson you can view the results and it tells you the words you got right and wrong, you can then further study these.
When I'm on the go and only have my iPhone, I will still ensure all my words are strengthened before starting the next lesson. I feel like it is a great refresher before I get into something I haven't learned or studied for yet.
The task is a little more tedious on the mobile version, mostly because if you lose all your hearts you have to start over, and that can become a little frustrating, especially when you feel like you should already know these words, and you're just strengthening them.
Remember though, this is exactly why you are strengthening them, because you might be slowly forgetting, and getting them wrong only proves that point. Soon enough though, as I have found especially with the easier words, I fly through them in strengthening.
EDIT: I would also like to add that I love the flash cards version for strengthening skills. I wish it were a part of the Duolingo app.
That is all I've got for today, thanks for reading :D
TL:DR - Strengthen all weakened words before moving onto a new lesson :)
Where are you finding weakened words on the iPhone app? I've only seen this section on the web page not the phone or tablet version.
At the end of each section there is the Strengthening words option. You're correct, the specific weakened words are only on the web page. Which is why I like to strengthen them on here.