After arriving on the Zuma Ridge fire road from Kanan, make a left and you'll see the singletrack appear on your right.
The trail begins with a short but very steep climb. It will turn into a very narrow, steep, and short descent that begins under cover of brush with a 90-degree sharp left at the bottom. The trail continues for about a half-mile with moderately technical and narrow sections until it dumps you out on Trancas Canyon fireroad. Continue up the fireroad (not down) for ~100 meters to discover the clearly marked trail on your left.
Now begins one of the best descents on the Backbone. It has a short uphill section near the beginning, but you can hit this with a lot of speed to mitigate the effect. The trail is pretty narrow with a fairly concerning drop on your left. There can be a lot of loose rock, so you need to pay close attention if taking this at speed. Also be mindful of hikers (rare, more on weekends) and very rarely horses (the local area is horse-friendly after all).
After about a mile of fast and, at times, sketchy descent, you'll cross a bridge into a very scenic forested area. Continue for a short distance and make a right onto a half-mile or so climb to Encinal, which features a large dirt parking lot and more of the Backbone Trail