The words I have marked as Known on LingQ, many of them I can't speak. I can recognise them 100% of the time, but I just can't pronounce them or even recall them at will. My Known Word count on LingQ is strictly a measure of how many words I can recognise. After some speaking/writing practice, you slowly turn that 'passive vocabulary' (i.e. able to be recognised) into 'active vocabulary' (i.e. able to be produced/spoken/written). LingQ focuses on only building up your 'passive vocabulary'.

I usually put I word down as known if I can undertsand it in context.

the entire premise of lingq's methodology is predicated on the difference between passive and active knowledge, corresponding to input and output. the more of the language you can passively understand, the easier it is to learn, the more resources available to you to learn from. so its almost a whole new process converting passive to active, whereas traditional classroom learning depends on trying to churn passive to active as quickly as possible, or even putting active before.

therefore i mark a word as known as soon as it would not cause me to slow down during the process of consumption, which is different from ability to use it actively. lingq does not train active knowledge, except i suppose with the forum for correcting written text. if you want to practice your output (speaking) you need to...practice speaking, anki, italki, conversation partners, etc. but since so much less of the language is necessary to communicate than to understand, albeit at a higher level of knowledge, i did not do much independent output training and my speaking is already far better than my listening, just because you need so much less of the language to begin practicing it, and because in live conversations you have so many clues from context and the ability to ask the person whatever you want