What:
The 'Greatest': An offensive player realises that they cannot catch the disc in-bounds and jumps from an in-bounds position, catches the disc and throws it before they land out-of-bounds. Result: This is a valid pass.

Why:
Rule 11.3.1. states that an airborne player who jumped from in-bounds is still in-bounds. This lasts until the player lands out-of-bounds and by that time the player has already released the disc. Therefore, the disc is not out. A player is allowed to throw the disc while in the air, according to rule 18.2.1.1. Extra: A player may not catch their own "greatest" throw. That is a turnover (13.2.5). If there is a contested call regarding the catch after a "greatest", the player who threw the "greatest" should be treated as the last thrower. They should establish a pivot at the point on the field closest to where they released the disc (or on the goal line if this would result in a pivot in their attacking end zone).