Tank Rengar is played pretty much the exact opposite of glass cannon Rengar. Patience, farming, split pushing, and bullying/forcing your opponent out of lane (as opposed to outright killing them) are all staples to it. Getting a 50-60 CS lead is pretty much the same as getting 3-4 kills.
I imagine it's not as fun as blowing people up the second you see them, but it's still funny seeing your opponents trying to catch a Rengar with a level advantage, 300 armor, 150 MR, 3000HP, and can heal himself every 5 seconds or something dumb like that.