In Off-roading a Spotter is the second set of eyes to help a driver navigate difficult obstacles. Spotters are usually in front of the vehicle and use voice commands along with hand signals to indicate which way to turn the wheels and whether to go forward or backward. When the driver cannot see what is directly in front of his or her wheels, it can make the difference between getting through or damaging the vehicle.

Ground rules 
 Once a driver agrees to have help from a spotter, only one spotter at a time should guide the driver. Others may watch, but should not interfere.
 An exception is that in difficult situations a second spotter may stand at the rear of the vehicle. His or her input is limited to "stop", "wait" or "hope".
 Hand signals are the best communication. Engine noise, others talking or other sounds do not interfere with hand signals. The spotter remains in clear sight of the driver and uses only those signals that were previously agreed to. 
 Signals should not be just pointing a finger. Signals need to be large, clear gestures that are not easily confused with other commands.

Basic commands/signals 
 Come forward! In a clear voice the spotter tells the driver to move the vehicle forward leaving the wheels pointed where they are. The hand signal is both hands up, palms held towards the spotter (away from the driver), gesture to move forward.
Turn driver! The spotter tells the driver to turn towards the driver's side (which is left in the US and many other countries). The words left and right are relative to the driver or the spotter who are facing each other and may cause confusion. The hand signal is pointing to the driver's left.
Turn passenger! The spotter tells the driver to turn the wheels towards the passenger side of the vehicle. The hand signal is pointing to the driver's right.
Stop! Is a clear command to halt the vehicle where it is and maintain control. The spotter should not use ambiguous words such as "OK", "that's good", "hold up" or "wait". The hand signal is usually both hands held up, palms toward the driver, and do not move the hands or fingers.
Backup! The spotter is telling the driver to put the vehicle in reverse gear and slowly back up keeping the wheels pointed where they were. The hand signal is usually both hands held up, palms towards the driver, moving the hands back and forth gesturing to back up. 

If spectators are giving the driver signals or shouting commands or advice, the spotter should step in and take control of the situation. Otherwise the driver will not know who's advice to follow. Other signals may be used as well as long as they are agreed to by both the driver and spotter.

Advanced spotting 
If a vehicle gets into a situation where one or more wheels are "hanging air" (off the ground and have no traction), the spotter may try to re-balance the vehicle. The most dangerous way is to hang on the side of the vehicle using the spotter's weight to bring it back down. Here the spotter is acting as a 'Rock surfer'. A much safer method is to use straps or ropes to bring the wheel or wheels down to earth again.

The spotter may be the agreed-on person to handle the winch line. Here again, clear signals between the spotter and driver need to be maintained in order to keep everyone safe.

References

Other websites 
 5 Jeep Spotting Principles, YouTube
 A Sampling of Uses for Winches and Spotter Lines for Off-Road Recovery, YouTube
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