Stay on the slickrock! The BLM has opened this place up as a rare Free Ride Mountain Bike area, please don't ruin it by riding through potholes, over vegetation or across patches of biological soil crust.
Follow the 4x4 track west up the wash for about a quarter mile until the mountain bike trail makes an obvious departure to the right, heading for what appears to be a wall of sandstone. This wall is in fact the start of the slickrock portion of the trail, though you'd be hard pressed to ride up it. For an easier route up, follow the wall to the right a few yards and find the bypass trail.
Once you're up on the sandstone, start making your way up and to the west, along the slickrock shoulder of the canyon wall. Dashes of white paint mark one possible path to the point where the trail crosses a sand dune. Follow the trail up and over the dune; you'll probably have to walk at least part of the way.
Beyond the sand crossing, follow the contour for another half mile until the slickrock expanse starts to open up a bit. You'll know it when you've reach the "playground" area at the western point of the mesa.
From here, it's all up to you. Explore around a bit and you'll find bowls, drops, wall rides, chutes and half-pipes...just pick your line and go for it. Be sure to scout them out first though, or it could have serious consequences.
Many riders will just play around here then head back the way they came, but if you want more slickrock terrain, you can keep going around the point. Following the slickrock terrace around the other side of the mesa, continue along the shoulder for another half mile before it pinches out and becomes unrideable. At that point you can either turn around and follow the slickrock back to the trailhead, or drop off the slickrock and down into the bottom of Hidden Canyon to loop back to your car along sandy 4 wheel drive roads.