Hi April:
We might first ask why students are disengaged in the first place?
But let us assume that it is occuring as is most likely from time to time. Simple techniques can be effective, and sometimes more aggressive moves are needed. Simple would be like saying, " Bob, did you know that...blah, blah, blah? That command everyone listenting to hear the teacher and what Bob is going to say.
Medium efforts might be to share a story or work related situation that has occured and students will re-focus - students like stories.
More aggressive is like changing from lecture… >>>