The key with nunchaku, flails, and the like is to not let your elbow bend enough that the business end can reach your head or trunk. As long as your arm is held out enough, you shouldn’t be able to injure yourself with a full-on hit. Power the swings by turning the whole body, keeping the arm out and relatively stiff.

(Naturally there are exceptions and nuances that are best explored with a teacher, I’m just giving a tutorial on how to safely experiment.)

“Be like water” is for rigid weapons like the sai; for flexible weapons like nunchaku it is advisable the body move as more of a solid unit.