@shincheckin
Don't know if you need any more tips, but it might help someone else too.
I'm not the best at it, and i don't really use it during fights, so others may have better pointers.
(I might or might not have used a sided variation against some dickheads in TKD, disguised as failed snapkicks)
The way I know it, there are 2 main types:
The simple one is to raise the knee, then hipthrust it forward. It's a just piercing knee but with the 2 movements
(up and forward) decomposed instead of both blended together.
Personally, the knee tends to drop when I hipthrust, so I got to raise it higher than where I aim. I am also not really good with the front one, so I use a small jumping step to add more power. But honestly, i dont think i ever used the front one a fight. With the rear one, i am more descent, but still, not my to go move.
You can use the raising of the knee as a feint, that will show you the opponent's reaction and continue accordingly.
You can also go for a teep/front kick when you raise the knee and realize the distance has changed.
I dont pull my adversary towards my strike as I would usually for the other types of knees, but I help myself to lunch forwards. Not sure if the difference is clear. After the contact, you can continue to "dig in" and pull the opponent's head down, to break their posture.
A lot of times, you will have to release the clinch to add more power, bringing your arms back.
For the second variation, the leg has an angle.
The big difference is that at the moment of impact, you also snap your leg forward, often adding a push to the strike. Depending of the timing, it can have as a result to either push, or strike with the upper shin part.
The snapping adds reach to the strike.
Also, at least for me, the balance changes when I add the push, so its something you will need to work with a partner. Some times you will find yourself able to put your leg back where it came from, but some times you will push the opponent and find your self needing to step down in a switching position.