When you drive on a slick surface the power from the engine will go to the wheel with the least amount of traction, so which ever wheel is slipping the most gets the most power. That's because the laws of nature, a.k.a. physics, tell us that force will always take the path of least resistance.
When an OHV is in four-wheel drive mode the front and rear axles are synchronized so there is always at least one wheel on each of the drive axles that can be driven by the engine's power effectively.
If you're in a 4x2 vehicle you can trick it into acting like a 4x4 by pressing the break peddle slightly to slow down the wheel that's spinning and transfer that wheel's energy to the wheel with traction.
4 x 4 (4WD) - Is a vehicle that has four-wheel drive (4WD). "4x4" in a 4WD vehicle means there are 4 wheels total and 4 wheels that are driven. Utility quads are typically 4x4.
4 x 2 (2WD) - Describes to a vehicle that has two-wheel drive (2WD) with four wheels. "4x2" in a 2WD vehicle means there are 4 wheels total and 2 wheels that are driven. The driven wheels can be either back or front wheels but are usually the back wheels. Sport ATVs are typically 4x2
Part-Time 4WD - Refers to an OHV that has a 4-wheel drive system which operates on-demand and powers all four wheels by synchronizing front and rear axles together via a shift lever. Part-Time 4WD usually includes two speed ranges, Hi and Lo.
Part-time 4WD systems have to be used in 2WD mode on pavement, cement or other hard, sticky surfaces. They are designed to be engaged only in specific situations when you need extra traction and damage can occur if driven on hard surfaces.
Full-Time 4WD - Refers to a 4-wheel-drive system that can be operated at all times on all surfaces. Full-time 4-wheel-drive systems usually have the option of part-time operation so you can shift to 2WD while on cement or pavement. Full-Time 4WD systems do not always have the Hi and Lo speed ranges.
Automatic Four-Wheel Drive (A4WD) - Defines the type of drive system that automatically turns on 4WD when it needs it. This is achieved with monitors that sense different wheel speeds then engage 4WD. The Polaris Ranger Electric Vehicle has this kind of automatic system.
Shift on the Fly 4WD - This 4-Wheel-Drive system allows the driver to manual shift from 2WD to 4WD Hi without stopping first. These systems typically have a speed limit at which you can engage the system; typically it's under 60 mph. OHVs that use an electronic actuator (like a push-button vs a shift lever) will only allow shifting to 4WD-Hi while under the rated speed, so pushing the button will not attempt to engage 4WD.
Vehicles with a shift lever may not know when they are going too fast to shift into 4WD Hi so doing this can cause damage. Consult your owners manual if you have an On the Fly 4WD system.
All-Wheel Drive (AWD) - Defines a full-time single-speed 4WD system that will supply power to all four wheels. Each system has a different front-to-rear power delivery ratio.