As Steven nods, Strange looks a bit relieved. Good. At least this way, he's got something close to a plan. He trusts Steven to relay the message somehow and he trusts Lambert to actually do the deed if the situation calls for it.

"Keep your eyes shut and remember to breathe," is all Strange says. A moment later, he turns into his purple smoke form and swirls around Steven's head. Even though he technically doesn't have eyes right now, Strange is intently watching Steven's breathing. The moment Steven falls unconscious or looks to be asleep, he'll float away and resolidify. After all, he just wants to knock Steven out, not poison him entirely.