Creating a 'lean effect' on the character?

Could anyone point me in the right direction for creating a 'lean' effect on the torso, that is, leaning backward or forward based on the user's mouse? It's the welding that's baffling me - I've tried playing around with the existing welds but none give the desired effect.

If you've ever seen those scripts that rotate the head on the X-axis, that's the kind of effect I'm aiming for, but the entire upper body.

Just to note, I'm using the R6 rig.