In this video Nathan Carnage Corbett explains and demonstrates the use of the Push Kick or Teep:
The next video is also a good demonstration of the Teep or Push Kick as demonstrated in a Muay Thai class. Notice how the back foot is flat on the floor acting as a solid and stable support for the push kick. There are a couple of variations of the push kick, like the inverted push kick but the idea is the same, pushing the opponent back and disrupting their flow of attack. It’s important to bring the kicking leg back in the same quick motion that you push it out, not allowing time for your opponent to grab the leg. Mostly it’s used as a defensive kick to stop an attack, but it can also be used as an effective weapon on its own or to push the opponent into a corner and off balance before unleashing a counter-attack.
Use a heavy bag to practice the push kick regularly, alternating legs from time to time. As a challenge try going one round continuously push kicking the bag and preventing it from swinging back towards you. With time and practice you should have a fast and powerful push kick.
If you want to incorporate push kicks into pad training, have the pad holder wear a belly pad as you see in the Thai Pad Training video.