I remember running a trail with a couple of friends, I was number 3 in line. I watched which lines they took and decided which one I was going to take and started. Half way through, I bounced off a rock, and lost my line. My friend started to "spot" me through a completely different line, and I eventually had to pull winch cable.
There is the difference between a good and bad spotter too. When you gain some experience behind the wheel, you will tend to need a spotter less. But there will always be an obstacle that will be best taken with an extra set of eyes. The spotter needs to know the capabilities of the rig, and how to properly spot.
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