Depends on how the card reads. Subzero Defenses is a Constant Effect, meaning as long as the conditions are met, the effect happens immediately.
Most cards that have the Sacrifice word on them usually begin with the bold word Action. This is a Triggered Ability, as described in the most recent FAQ update.
I'll quote the Rulebook and FAQ here as it seems appropriate.
The original section on "Constant Effects" from the rulebook
Many cards simply announce an effect, with no bold
precursor. Such effects, called constant effects,
become active as soon as their enabling card comes
into play and remain active as long as that card is in
The objective card “Jedi
Training” is an example
of a card that provides a
constant effect. As long as
this objective card is in play,
it gives one additional Force
icon to the LS player during
each Force struggle."
Here is the most recent FAQ on Triggered Abilities
"(4.13) Triggered Abilities
Action, Reaction, and Interrupt abilities are known
as triggered abilities. Such an ability is used when a
player optionally initiates it at a specific moment in the
If the word “Forced” precedes a triggered ability, the
ability’s initiation is mandatory"
In an instance where a Constant Effect and a Triggered Ability would potentially happen at the same time, I believe the Constant Effect is always resolved first.