Lightning Kick (稲妻キック, Lightning Flash Kick) is the name for Zelda's forward aerial and back aerial attacks, as confirmed by one of her trophies in Super Smash Bros. Melee. When sweet spotted, the Lightning Kick pauses the opponent in place for a second dramatically (except in Melee), before dealing heavy electric damage and launching them away with very powerful horizontal knockback. A sourspotted hit on the other hand, delivers very low horizontal knockback with no special effects and much less damage.
Between Melee and Brawl, the attack was buffed in terms of knockback when sweetspotted, along with having the aforementioned freeze frames added (despite this, the sweetspot also became impossible to SDI with the addition of a 0x SDI multiplier), though the sourspotted kick was weakened further, to where it deals less than half the damage it did in Melee, with 4% compared to 10% (which does make it a much better tool to lock opponents on the positive side). The sweetspot in Brawl is significantly more difficult to land, only lasting for 1 frame as opposed to 4 in Melee, and being about a third of the size. Both kicks also have slightly more ending lag making it impossible to perform two forward Lightning Kicks in a short hop.
In all games, a sweetspotted Lightning Kick is the strongest forward and back aerial, being among the few aerial attacks that can reliably KO under 100%. Along with this great power, both forward and back aerial Lightning Kicks are fairly fast in start-up lag, while having good reach. However, in Melee, both Lightning Kicks can be difficult to sweetspot with Zelda's low mobility along with the spacing requirement to avoid the flub hitboxes, which limits the usefulness of the aerials. In Brawl, the much smaller window of opportunity to land the sweetspots leads to even more difficulty in landing them.
The forward and back aerial versions are very similar appearance wise and statistically. In Melee, the back aerial Lightning Kick is slightly better due to slightly faster start-up lag and slightly stronger knockback. Brawl adds electric properties to a sweetspotted down air, making it a "Lightning Stomp" of sorts.
Between Super Smash Bros. Brawl and Super Smash Bros. 4, the Lightning Kick was slightly nerfed in terms of knockback when sweetspotted. While a sweetspotted Lightning Kick is still arguably the strongest forward and back aerials, its base knockback is decreased a bit, KOing at slightly later percent. The knockback growth/scaling however, remains the same and can still KO characters under 100%. It also has significantly increased ending and landing lag and can not be auto-canceled after a short hop making it a much worse approaching tool. Animation-wise, for the forward version, Zelda performs a back spin kick (via her left leg) with her crossing-right arm somewhat-bracing her face. For the back version, Zelda kicks with her left leg instead in a similar motion, but without the turn.
In Smash 4, however, Zelda has different animations for both which also very slightly delay their startup; for the forward version, Zelda recovers in a full body spin just after kicking, while with the backwards version Zelda twists her body somewhat then thrust-kicks behind her; both versions also involve her arms being positioned differently as well as her hands being clenched more into fists and also have her both lean and face away from her kicking-directions more. Also due to the change in hitbox placements, the attacks are also easier to connect with; though the new animations also somewhat alter her hurtboxes but give them slightly less start-up.
Between Super Smash Bros. 4 and Super Smash Bros. Ultimate, forward Lightning Kick now comes out as fast as the back version and the back version is marginally stronger. Both versions also has less ending lag, less landing lag and auto-cancel earlier although they still cannot auto-cancel in a short hop.
The Lightning Kick is very similar to Captain Falcon's forward aerial, the Knee Smash, as both are effective aerials that deal very powerful horizontal knockback with electric damage when sweetspotted, while being exceedingly weak with no special effects when sourspotted. However, the Lightning Kick hasn't reached the same level of notoriety as the Knee Smash, where the latter was more effective and prolific in competitive play for Melee. The main reason for this was that the knee was notably easier to sweetspot, as sweetspotting it only required landing the initial hitbox as opposed to landing a specifically spaced hitbox in the attack, and combined with Falcon's high mobility and great combo ability, it has seen much more utilization. Zelda's poor tournament viability and general lack of Internet notoriety compared to Captain Falcon also hindered the Lightning Kick from receiving as much attention as the Knee Smash.