Mandatory - Optional effects

Mandatory effects are effects that have to happen. They don't give you a choice. Nor will they miss timing. Examples are;
Sangan:

Spoiler:

Ally of Justice Catastor:

Spoiler:

Optional effects are effects that you can choose to use. There are two different versions of optional effects, the If effects and the When effects.

Optional If:
Neo-Spacian Grand Mole:

Spoiler:

Optional When:
Black Rose Dragon:

Spoiler:

If effects don't start a chain, since they only care that the requirement is met, for example, "if this card is summoned". in that case, it only cares that it's summoned, and not at the time that it is summoned. it will simply start a new chain after the old one resolved.

When effects miss timing when they're not the last thing to happen in a chain. This is due to the fact that it says "when this card is summoned", meaning, at the exact time it's summoned, it's effect activates. and when it happens while a chain is resolving, you can't use those effects, because you can't chain during the resolve of a chain link. hence they miss timing, and cannot be used.

An example of missed timing:


Lightpulsar Dragon:

Spoiler:

If Lightpulsar is destroyed by:
Soul Taker:

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Due to the lifepoint gain happening directly after the destruction and the gain being a mandartory effect lightpulsar misses the timing and cannot summon a Dark Dragon.

mhm, one of many possible scenario's. Still, King of miising timing is and always will be Peten, The Dark Clown