Spell Speeds:

In Yu-gi-oh there are 3 well known Spell Speeds:

Spell Speed 1 (Spells [Normal, Equip, Continuous, Field, Ritual], Effect Monster's Effects [Ignition, Trigger, and Flip])
Spell Speed 2 (Trap [Normal, Continuous Initial Activation] Quick-Play Spells, Effect Monster's Quick Effects)
Spell Speed 3 (Counter Traps)


In regards to activation you may only activate a card in a chain if the spell speed is equal to or greater than the card before it



But there are also cards that somewhat ignore these rulings and have their own Spell Speeds, these are the "Special" Spell Speeds:

Spell Speed 2.5 (Continuous Effects [Spell/Trap [After Activation]/Monster[After Summon] ] )

Spell Speed 4 ("Spells/Traps/Effect Monster's Effects cannot be activated in response to this card's activation/summon")


Difference between 2 & 2.5:

- Refer to this lesson on types of effects.
Lesson: Spell/Trap/Monster Effect Types

To sum it up there is no activation of a continuous effect.

Example:
Jinzo is normal summoned successfully. No traps can be activated in response to its summon except counter traps because of spell speeds.

Traps (2) < Continuous Effect (2.5) < Counter Trap (3)
Trap Hole < Jinzo < Solemn Warning

Now consider other Continuous effects that ALLOW an activation of cards such as Skill Drain. Cards like these are continuous effects to negate a card, but it more or so happens upon resolution and not in the chain itself.

Ex.
Summon Caius the Shadow Monarch while Skill Drain is face up targeting it.

Chain Link 1: Caius the Shadow Monarch

Upon Resolution the Effect of Caius would be negated, but let's add another chain.

Chain Link 2: Book of Moon. Targeting Caius.

This is a sneak preview of the lesson ahead but for chains they work in reverse.

So Caius would be flipped facedown 1st. Then the effect would resolve because Skill Drain affects Face-up Monsters upon resolution.
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Chains:

In regards to chains there are 2 Golden Rules:


Rule 1:
All Chains activate sequentially, but resolve backwards.

What this means is this:
1. A Chain goes up in number +1 for every chain.
2. Once no player wishes to continue a chain, the chain resolves backwards starting at the highest numbered chain.

Rule 2:
In the Damage Step, Attack/Defense Modifiers may be ACTIVATED, but only Attack/Defense Modifiers & Spell Speeds 2.5+ may be CHAINED.

This means this:
- Spell Speed 2 Cards that Modify Attack and Defense may be activated in the Damage Step.
- Spell Speed 2 Cards that Modify Attack and Defense, Continuous Effects, and Counter Traps may be Chained.

Rule 2 can get complicated with the 2nd half so allow me to explain with an example.

Continuous Effect Example: Fabled Unicore

ex.
- Fabled Unicore is face up on the field and Elemental Hero Alius is also face up.

Unicore Player: 4 Cards in Hand
Alius Player: 5 Cards

- Alius Attacks Unicore

- In Damage Step: Honest is activated and sent to the Graveyard

Chain 1: Honest

Resolution: Unicore Negates Honest, Alius remains at 1900 Attack.

- Alius is Destroyed and Player loses 400 Life Points.
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As for Counter Traps if they activate a spell speed 2, then a counter will negate it.