When and why is it correct for a support to use flicker defensively for another player?

It is correct for a support to use Flicker defensively for another player when that player is a core damage dealer (marksman, mage, or assassin) who is about to be killed by a high-burst damage ability or a crowd control chain, and when using Flicker will reliably save their life, allowing them to continue contributing to the team fight or secure a crucial objective. This decision must be made quickly and strategically, considering the relative value of the support's Flicker versus the core hero's continued presence in the game. The primary condition is that the core hero is in immediate danger of being eliminated. This could be due to a coordinated enemy dive, a high-damage ultimate ability, or a chain of crowd control effects that prevent them from escaping. Secondly, using Flicker must guarantee their survival. This means that Flicker must move them out of range of the immediate threat and provide them with enough time to reposition safely or receive further support from their teammates. Finally, the core hero must be a significant asset to the team. Sacrificing Flicker to save a hero who is already significantly behind or who is unlikely to have a major impact on the game is generally not a worthwhile trade. An example would be using Flicker to teleport a marksman away from an assassin's ultimate ability, allowing them to survive and continue dealing damage in the team fight, potentially securing a victory. However, if it would only delay their death by a second or two, it isn't worth it.