Here's why:
* Hieratic Dragon of Tefnuit's effect: When this card is sent from the field to the Graveyard, you can Special Summon 1 "Hieratic" monster from your hand or Deck.
* Gustkrake's effect: You can Ritual Summon this card with "Hieratic" monsters as Materials.
* Timing: The effect of Hieratic Dragon of Tefnuit triggers when it is sent to the graveyard. This happens *before* the Ritual Summon of Gustkrake is completed.
Therefore, you can activate Hieratic Dragon of Tefnuit's effect to Special Summon another "Hieratic" monster *before* the Ritual Summon of Gustkrake is complete.
Important Note: You can only activate the effect of Hieratic Dragon of Tefnuit once per turn.
Let me know if you have any other questions!