Edison Rules Compendium

attacks

At the end of the damage step (substep 7):
If the defending Machina Fortress was destroyed, these effects can form a chain!

Goyo Guardian won't trigger if either monster left the field before Substep 7, as it states "And sends it to the GY".

●The attacking Goyo Guardian's effect is optional, so it is Chain link 2.
●The defending Machina Fortress' effect is mandatory, so it is Chain link 1.

You can view the section on "SEGOC" for more information regarding the order of these chain links. View SEGOC section