The official Nephi's Twist trail is a doubletrack heading up a wash from La Verkin Creek to the Moenkopi Trail. It's about 1.5 miles long, and has a ridiculously steep section near the upper end. If you're doing the official ride, I'd suggest downhill only, or do it as an out-and-back from the bottom doing only the first 0.8 mile.
The Nephi's Twist singletrack is an experts-only romp along a ridgeline of Moenkopi clay. It starts on a spur off the Moenkopi Trail and connects to Nephi's Twist DT about 0.7 miles uphill from the bottom trailhead. The fun stuff is 3/4 mile in length. The ride can be done as a 3-mile DH with shuttle (or with a 3-mile road return), or as a 2-mile loop (which would include a stiff climb vs push-a-bike).
Nephis Twist, Toquerville, UT 84774