Optional Monster Effects are Trigger Effects that don't happen automatically when the trigger condition is fulfilled - the controlling player decides whether to activate it or not. These effects involve cards like "Mystic Tomato", "Peten the Dark Clown", and "Dark Magician of Chaos".
It's possible to miss the timing with Optional Trigger Effects, unlike Compulsory Trigger Effects. An Optional Effect "misses the timing" when the triggering condition isn't the last effect to happen in the game state, or didn't happen in the last Chain Link in the current chain.
Most of the time, Optional Trigger Effects can be identified by observing the words, "When...you can..." in a monster's effect text.
On resolution, "Raigeki Break" resolves first, destroying "Peten the Dark Clown". His condition has been met, but can't interrupt the Chain. "Fissure" resolves next, destroying one of Player B's monsters. Now the Chain is complete. However, since "Peten the Dark Clown" being sent to the Graveyard was not the last thing to happen, Player A has missed the timing to activate its effect.
Tributing monsters with Optional Trigger Effects for a Tribute Summon also prevents their effects from activating. The last thing to happen is the Summon of the Tribute monster, causing the timing window to close immediately.
Compulsory Trigger Effects will never miss their timing, as they are mandatory. They lack the words "you can" in the effect description, meaning the effect must attempt to activate eventually.
On resolution, "Raigeki Break" "resolve"s first, destroying "Sangan". His condition has been met, but can't interrupt the Chain. "Fissure" resolves next, destroying one of Player B's monsters. Now the Chain is complete. Since the effect of "Sangan" is mandatory, it activates now, and forms a new Chain with its effect as Chain Link 1.
A common question is, "What happens when two Optional Trigger Effects try to activate at the same time?" For example, two copies of "Mystic Tomato" destroy each other in battle. Neither will miss the timing, as the last thing to happen was both being sent to the Graveyard. Both of the effects trigger at the same time, and form a Chain. The Turn Player's effect becomes Chain Link 1, and the opponent's effect becomes Chain Link 2.
Only the activation of an Optional Trigger Effect can miss the timing. In other word, as long as the effect is activated, the resolution will never miss the timing. So two monsters will be summoned in the above example.
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK - DEF - Level - Attribute - Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua - Beast - Beast-Warrior - Dinosaur