Asanas are the building blocks for a kriya. Asana is a posture. It is an exercise which addresses specific pressure points in the musculature to enhance the functioning of glands and organs of the body. Every posture is a dynamic transformative creation which links you with your infinite self. It brings consciousness to a specific body area, it makes us aware. Meditation also affects the emotional state and connects mind with body. It acts as a stimulus to channel energy through the meridian points.
Kriya is a sequence of postures (asana), breath (pranayama), and sound (mantras) which lead to a specific desired outcome or state of consciousness when practiced regularly.
Bhanda is a body lock which is to serve as a control switch for balancing prana and apana. We have three locks in Kundalini yoga: 1) Neck lock (jalandhar bandh); 2) Diaphragm Lock (uddiyana bandh) and 3) Root lock (mulbandh). The great lock (mahabandh) is when all three locks are applied.
Mudra is a hand position, which is specific to channels of energy and sends specific signals or reflexes to the brain. It is like a switchboard with which we connect to each planet and its qualities. We usually, but not always, touch the thumb to any of the other fingers to create a specific mudra.
Breath or Pranayama
Breath is a fundamental building block in Kundalini Yoga, as it is used to promote health and vitality, to control the mood, and develop concentration. Awareness of the breath combined with sound gives us the ability to creatively direct our lives and open up our unlimited potential. Breath is both gross and subtle, taking its form both physically and non-physically (as prana, the subtle life force energy).
Pranayama is the science of breath; it is a form of energy management. The mind follows the breath. If you want to control your mind, you need to learn how to control your breath.