I see so many posts about reverse trees but what are they exactly plz ?
Taking French as an example: You would change your UI language to French, and then start the English for French speakers course. This gives you a bit more vocabulary, and the opportunity to respond more in French. Also access to the forum/topic for that course, where you can practice your French whilst helping English learners, if you want to. [Tip: Turn off speaker and microphone in account settings to remove most of the irrelevant exercise types]
Another term used is 'laddering' where you would do the German for French speakers course instead. This allows you to reinforce your French knowledge, whilst learning German. Some claim that this also helps to avoid language confusion. Afterwards, you could also do the reverse tree, which in this case would be French for German speakers.
In addition to reinforcing your learning by requiring you to respond more in the language being acquired, both "reverse trees" and "laddering" are valuable because they provide fresh exercises. I find the standard exercises can rapidly become a little limiting because I can easily memorize them, without developing the ability to independently use the underlying grammatical principles.
E.g. my normal tree is English to German, since I'm English and I'm learning German (hence level 24 German).
My reverse tree is German to English, which I find much trickier, since I need to give most answers in German, rather than in English. (I've reached level 6 in English, but really these are just sort of extra levels in German, since I already know English)