Missing the Timing

A mandatory trigger ("When …, …") and an optional trigger ("When …, you can …") behave differently when their trigger condition is not the last thing to happen in a chain or sequence.

The rule

• Optional "When … you can" — misses the timing if its trigger is not the last thing that happened. The effect cannot activate.
• Mandatory "When …" — does not miss the timing. It joins SEGOC at the next available window.
• "If" trigger text — does not miss the timing because it is not strictly checking the last action.

Examples

1. if turn player activates Dracotail Lukias‘s on-summon effect and then chains their Dracotail Gulamel to target the opponent’s Elemental HERO The Shining, Elemental HERO The Shining misses timing for its optional trigger ("When …, you can …") effect because the last thing that happened was the turn player resolving Dracotail Lukias‘s effect to add a "Dracotail" monster from their deck to the hand.
2. Pre-errata Sangan with "When … you can" used to miss the timing because Tributing for Caius leaves Caius on the field, and Caius's effect to banish is the last thing that happened.

Quick check

Ask: "What is the last thing that happened?" If it is the trigger event itself, the optional effect can activate. If anything else happened after (banishing, special summoning a different monster, applying a continuous effect), the optional effect misses the timing.