According to Honda service information, the rear clutches are modulated by varying the amount of
current to the coils using battery voltage.
There has been a ton of speculation on the VTM-4 clutch holding capacity and consequential silent failure, but I would suggest that my results simply do not show any sign of silent failure.
Since your post, Honda revealed in an
article on the 2019 RDX that the 4th generation SH-AWD system (same hardware as iVTM-4) can handle 3,000 N·m which is 2,213 foot-pounds.
The G2's engine develops 262 foot-pounds of torque and the overall drive ratio in first gear is 14.276:1 resulting in a maximum torque of 3,740 foot-pounds which becomes 3,641 foot-pounds because of the 2.7% overdrive inside the iVTM-4 unit. Honda states that "up to 70% of available torque" can be sent to the rear wheels. 70% of the available torque would be 2,549 foot-pounds. Typical drivetrain losses are 10-15%. A 13% drivetrain loss would result in 2,213 foot-pounds which is exactly the limit Honda stated. I think we've found our answer - any amount of torque above 2,213 foot-pounds being applied to the rear wheels results in clutch slip.
As to how this applies to the G1... Honda stated in the same article that the 4th generation SH-AWD (iVTM-4) improved torque capacity by 40% over the previous version which was limited to 2,200 N-m or 1,623 foot-pounds. The G1 produced 247 foot-pounds of torque and the overall drive ratio in first gear was 3,019 foot pounds or 2,969 after factoring in the VTM-4 unit's lower 1.7% overdrive. 70% of this number would be 2,078 foot-pounds. Assuming a 15% drivetrain loss, the torque to the rear wheels would be 1,766 foot-pounds which is reasonably close to the 1,623 foot-pound limit (Honda did say "around" 2,200 N-m in the article).
I think we've found our answer - any amount of torque over 2,213 foot-pounds in the G2 or 1,623 foot pounds in the G1 being applied to the rear wheels results in clutch slip. No wonder the G2 runs off and leaves the G1 in the dust!