you know it; you get it
It is used (as a casual response) when someone shares with you something very specific and niche (or, expresses their taste/liking for such things), and you strongly and emphatically agree with them.
Note that it sounds a bit masculine and bossy.
I love to drink Japanese sake with iburigakko (smoked pickled radish) and cream cheese.
Yeeees, you know it!