I do my absolute best to avoid using it.Usually I'll leave it to the CT to translate. I will try stick figures, miming, and repetition if there is no CT to help me out. As the last resort I might write the Korean on the board, but I try not to say it out loud.
Outside of class and outside of school, I will say small things in Korean. They know I'm studying it and I only reveal my Korean to kids I already have a good relationship with.
I have multiple reasons for this. One, you lose face when you make a fool of yourself trying to speak a language you can't. Now, I know this is what we're asking them to do, but you must understand we judge our students by western "it's ok" standards and they judge us by Korean "die before suffering embarrassment" standards. It's easy to lose their respect by speaking Korean in the classroom, and you're opening yourself up to ridicule.
Two, if the CT is there and you're speaking Korean, it's like you're rendering them useless. In my school, their primary job when co-teaching is to translate.
Three, students often assume you know more than you do. They will assume you understand them and will begin speaking to you in Korean more often than English.