Yoga blocks are used as props to help you balance in a challenging pose or for other asanas, or postures, where they become small stools. Rectangular blocks were the norm for many years, but there are other shapes available. Yoga blocks can be made from wood, cork or a lightweight plastic foam. Each has advantages.
Foam blocks are lightweight and best when you don't want to use a heavy yoga block. For example, your yoga teacher might ask you to do postural work by placing a lightweight foam block on top of your head. By extending through the crown of the head, a foam yoga block gives your skull a manageable weight to work against. This gives you feedback, so you are able to feel the action of that movement. Take the block away and it is more difficult to feel yourself do the same movement.
Foam Block in Bridge Pose
Lightweight foam yoga blocks are especially effective when you use them to teach internal rotation of the thighs in certain yoga poses. For example, in setubhandasana, or bridge pose, you can work internal rotation of the inner thighs by placing a light foam yoga block between the inner thighs. Rather than allow the legs to splay open, creating a less-stable position for the lower back, using the foam yoga block will engage gluteus medius muscles of the side hips. This will keep your sacrum physically stable in the pose, reducing your injury risk.
Heavy Cork Block in Supported Bridge Pose
Heavier cork yoga blocks are efficient when you need greater heft so your block doesn't slide out from underneath you in other postures. For example, in doing a supported backbend, you could set a heavier cork block under your lower back. Continue to draw the inner thighs downward. In this case, the cork yoga block is a soft surface that will not jam the small sacroiliac joints of the sacrum that protrude, and remain stable enough to not budge.
Cork Blocks for Downward-Facing Dog
Many yoga postures require upper-body strength. For some beginners, this is a challenge. Using heavier cork blocks angled at the baseboard of a wall allows you to move into adho mukha svanasana, or downward-facing dog, with less physical effort. Take each block and tilt it against the wall so it creates a 45-degree angle to the floor. Place your palms flat on each block and step your feet back into the posture. Cork yoga blocks are heavy enough to slide out from under your hands in this and other poses. Experiment carefully with different types of yoga blocks to see which ones are most appropriate for your practice.