Disclaimer: I am not an expert. The following is just my personal opinion/experience.
I've done muay thai/boxing on and off for about 2 years now. If you've seen me on the thai pads or the heavy bags, you'd know I am a beast (brief moment of cockiness). But my skills on the pads/bags do not translate well into sparring. Not because I am not physically capable, but because I am not
mentally capable. Dealing physical trauma onto another human being is a scary thing. And in most cases of sparring, that human being happens to be your friend.
So why would I want to hurt my friend, especially during training?
The old adage goes "you fight as well as you train". Remember that when you're running force on force drills. When the buzzer sounds, treat your friend, training partner, etc. as a hostile. If you're a good friend/training partner, you'll give him ~100% resistance, and he should do the same. Make he/she fight for the wristlock/disarmament, and vice versa. It's nothing personal, and if you two care about each other, you won't slack off. That being said, don't go snapping fingers and joints
.
If you're not hurting him in training, you're hurting him (by not adequately preparing him for real life situations). Why would you want to hurt your friend?
P.S. Play safe and use proper equipment at all times.