What does rule 540.1.b.1 mean regarding playing an additional spell when starting a chain?

Ruling: A player can hold priority and play more than one spell in a row when they are the first active player after creating the chain.

Sequence:

• Play an action spell to start the chain
• Hold priority and immediately play a reaction spell
• Other players get priority after both spells are played

Nuances:

• This only applies to the first active player after creating the chain
• The second spell is additional to the one that started the chain