In Four High (4H) mode, the center multi-disc differential (inside the transfer case) is locked, locking the front and rear propeller shafts together for a permanently locked front:rear 50:50 torque spilt.
Due to this "locked-up" nature, Four High should NEVER be used on pavement, not even on wet pavement. Torque windup and drivetrain binding will occur, which will shorten the life of the four-wheel drive system. Under prolonged binding, drivetrain failure will occur.
Sometimes people will describe Auto mode as "its like a automatic Four High. Under slip it auto engages Four High", or some similar description.
This is rather incorrect as Auto mode is a bit more complex than that. The front axle, front differential, and front propeller shaft are always engaged in Auto mode, whether they receive torque or not.
Also, torque is sent to the front wheels in 10 percent increments (10, 20, 30, 40, up to 50 percent - 100 percent in newer versions) based on tractive conditions and wheel slip.
The front:rear torque split in Auto mode (for the BorgWarner 4406 transfer case) is variable from 0:100, 10:90, 20:80, 30:70, 40:60, up to 50:50 (lock). As the center multi-disc differential allocates the engine’s torque output, increasing the percentage of torque sent to the front wheels, the percentage of torque sent to the rear wheels is decreased. The torque split is also adjusted or "fine-tuned" if wheel slip lessens or worsens while torque is being transferred.
Auto mode will not keep the 50:50 torque split very long, to guard against torque windup and drivetrain binding while on pavement.
From 2003 to present (BorgWarner 4416 and 4417) the ControlTrac system can send 100 percent of the engine’s torque output to the front wheels – for "temporary front-wheel drive". This capability of putting 100 percent of torque to one axle or the other is called "torque biasing".
The front:rear torque split in Auto mode is variable from 0:100, 10:90, 20:80, 30:70, 40: 60, 50:50, 60:40, 70:40, 80:20, 90:10, up to 100:0.
ControlTrac four-wheel drive is not you’re grandfather’s 4x4 system. It has a computer control system with dedicated microprocessor, and advanced control software that is artificially intelligent.
In Auto mode, it thinks and learns. Its also monitors your driving input and from 2003 onward, it can also predict traction loss before it happens.
Even when you think its driving along sending torque to the rear wheels, it will periodically supply torque to all four wheels. In this respect, its very much a full-time system.