Just a second is a variant of split second. Announcing Slaying Mantis instantly locks all permanents in place on the battlefield. As long as Slaying Mantis is on the stack, players can't cast spells, activate abilities, or move their permanents.

Slaying Mantis must be thrown onto the battlefield from at least three feet away. That is, three feet is a horizonal distance, not a vertical one.

No player may interfere with the throwing of Slaying Mantis, including while it's in flight.

If Slaying Mantis lands on multiple creatures, it fights each of them one-on-one. For example, Slaying Mantis lands on a 3/3, a 4/4, and a 5/5. Slaying Mantis deals 6 damage to each of them, destroying them all. At the same time, they deal a total of 12 damage to Slaying Mantis, destroying it. What a maneuver!

Slaying Mantis will fight any creature an opponent controls that it touched as it entered, even if it didn't end up touching that creature when it stopped moving.

If Slaying Mantis touches the object used to represent a creature token an opponent controls, it will fight that creature.

If you create a token that's a copy of Slaying Mantis, it also enters via flying elbow. However, you must use a Magic card-sized object to represent the token. As Slaying Mantis's just a second ability won't apply, players can move around their permanents at any time, including while Slaying Mantis is in flight. So that's fun.