AUTO_SKIP blocks can eliminate the delay between play time and resolution time, usually on a condition, for abilities in which there is usually no point in waiting for.
Firebreathing is a good example because usually there is no point in responding to a firebreathing ability as it doesn't increase a creature's toughness.
forced_skip will always eliminate the delay between an ability's play time and resolution time. This should be used only when it is required that there be no delay. Split Second spells and abilities are a good example as are mana abilities.
Both methods should be used with caution as the lag allows players to respond to spells and abilities (buffing a creature, countering a spell, flickering a permanent, etc...). So make sure there is a valid reason to remove the delay before using one of those, not just a "I don't like the delay so I'm going to take it out" reason.