What is missing the timing in Yu-Gi-Oh?

Answer 1

A card misses the timing when it is an Optional Trigger (they say 'you can') and something is summoned or resolves in between the trigger condition being met, and the first opportunity to activate it.

Peten the Dark Clown has an Optional Trigger that lets it summon another when it is sent to the graveyard. However this sending must be the last thing to happen, or you miss the timing and can't activate the effect.

So, if you tribute it for a tribute summon, it misses the timing due to the summon. If you send it to the graveyard as a cost, then it misses the timing due to the resolution of that effect. If it is sent to the graveyard at chain link 2, then it misses the timing due to chain link 1, etc. It needs to be sent at chain link 1, or destroyed by battle.