Ein Trigger

Ein Trigger is one of Ks Special Moves, debuting alongside him in The King of Fighters '99''.

Description
K' moves his right arm in front of him and forms a circular flame. This move is good for spacing, and sets itself up for many of K's several follow-up moves:

Strategy
One of, if not the most versatile moves in all of KOF, this is the bread and butter of K '' s gameplan.

By itself, the move can be used as a combo ender, a space control tool, an anti-projectile tool, or to whiff and build meter with. In KOF XV, the version knocks down on hit while the  version keeps the opponent. Whatever the case, once you successfully nullify the opponent's incoming projectile, K' can retaliate with a myriad of follow-ups.

KOF XIV introduced the Second Whip, an overhead follow-up that is probably the least useful, though no less helpful, of the follow-ups. It can be used to surprise an overly defensive opponent up close and, in KOF XV, juggle the opponent in the corner. The downside to this one, however, is its range, as it's not a very effective punish after successfully nullifying an incoming projectile. Still though, this is excellent combo filler and a good alternative to K '' s Knee Assault, his other overhead.

Second Shot, as the name implies, has K' fire off the flame by kicking it at the opponent. This can be used either as his main zoning tool (being able to challenge the likes of Ash Crimson or Athena Asamiya in that respect) or a punish if he absorbs an incoming projectile beforehand. Its main downside is its rather lackluster damage compared to other projectiles in the game (doing a rather poor 40 damage for the base version, compared to Ash's Ventose doing double the damage at 80) as well as the fact that he has to do Ein Trigger first before he can use Second Shot, wherein (unless he times the initial Ein Trigger properly to the incoming projectile) the opponent will have already hit him with their own projectile by the time he gets it out.

Another new follow-up added in KOF XIV, Second Knuckle is a very solid approach tool. Boasting the best horizontal range out of all his follow-ups, this can be a very strong fireball punish after K' absorbs the projectile with the initial Ein Trigger. Additionally, it's also a very solid combo ender since it's the most damaging of the four follow-ups from Ein Trigger. Compared to the other follow-ups, this is also the riskiest follow-up to use due to the slow recovery on whiff as well as being unsafe on block in most cases (unless you go for the version, which is the safest option). It should also be worth noting that the move is Super Cancelable, further adding to its damage potential.

Finally, Second Shell is an excellent combo starter that boasts the best vertical range of the four follow-ups. Consequently, it's also a safer anti-air alternative to Crow Bite as K' remains stationary even if the move whiffs. On hit, the move will launch the opponent (much like Kyo Kusanagi's 75 Shiki Kai), allowing K' to follow up with whatever option you prefer (usually a Super Cancel or a Minute Spike into a Narrow Spike depending on the screen positioning). KOF XV also adds the ability to perform a Blackout after an Second Shell, thereby increasing the move's combo ability even further.

Speaking of which, K' can also go straight into a Blackout after the initial Ein Trigger, which he can use to creep closer to the opponent or, if he's already close, cross them up and go on the offensive. It's best to use this on whiff or after absorbing a projectile, however, since this is very unsafe on block.

Trivia

 * Second Spike and Second Knuckle are functionally similar to each other. Likely suggesting that the latter is a spiritual successor to the former.