This timing is before the Summon response window. If the monster's Summon was negated in the Summon negation window, there will be no Summon response window afterwards. Only cards that negate Summons can start the Chain during this window, so "Mystical Space Typhoon" cannot be activated as Chain Link 1 of the Summon negation window, but it can be activated as Chain Link 2 if a card like "Royal Oppression" was activated first. Cards like "Bottomless Trap Hole", "Trap Hole", etc. that respond to the Summon of a monster cannot be activated during this window; they can only be activated in the Summon response window.