An A-skip is a running drill that is used as part of a pre-run warm-up routine. It focuses on running technique by focusing on your leg position, posture and foot placement. It also improves arm to leg coordination.
Start in a standing position with feet hip-distance apart, look straight ahead, and keep your upper body tall. Raise your left knee to hip height while pushing up on to the ball of your right foot. The aim is to hit the ground below your center of mass with the ball of your left foot and push off with your midfoot. Continue alternating sides and moving forward. Just like when running - your arms should follow your leg movements, remember to keep those elbows tucked in. Foot position and ankle stiffness is also important - keep your foot bent towards your shin (i.e. in a dorsiflexion position) and push off the ground with purpose!
To start with, slow down the movement and focus on getting each position of the movement right. This is to get you thinking about your running technique, the aim is not the height of the skip!