Brad's Iyengar Yoga Notebook
Passive, relaxed backbending over a bolster or rolled blanket is a good way to move into backbends and also a good way to cool down from a back bending session.   It is also useful as a counterpose after a session of forward bends, or as a general uplifting restorative pose.
Practice placing the bolster at different places under your upper back, solar plexus, lower back, and hips.   Breathe into the part of your body that is elevated by the bolster.
After doing the backbend across the bolster, bend your left knee and place your left foot on the floor close to your left buttock.   Use it to roll yourself onto your right side for a supported stretch of your left (upper) side.   Pull your left arm overhead with your right hand around your left wrist.   Extend through your left leg firmly.   Breathe into your left side and feel your breath expand your left side.   Then roll back up, and then onto your left side to repeat the stretch on your right side.   Next, rotate back upward and slide back so the bolster is under your tailbone.   Bend your knees into your chest and relax in this position for a while.
Come out of this pose by pressing your body up into a bridge, removing the support bolster manually, and then letting your back down to the floor.   Finish by resting on the floor on your back with your heels brought in as close to your buttocks as possible, knees in the air, hands on your chest, elbows on the floor.
Another prop that is useful for supported backbending is the wooden block.   You want to use the wooden block under your thoracic spine, not your lumbar spine.   You can use the block placed spinewise or sidewise on its low, medium, or high aspects.   Each of these variations will yield a different intensity and effect.   Experiment with placing the block at different places under your thoracic spine and at different heights to feel the effect of each variation.