In 2012, 80% of Greece’s private creditors agreed to “voluntarily”
convert their Greek debt into debt of a bit less than half the face-value (plus
a lower interest rate). With such a proportion having agreed to the swap without
triggering credit default swap insurance payouts, Greece could get the E.U. to
agree to force the remaining 20% to involuntary write-downs. That would trigger the credit default swaps,
at least in theory.
The full essay is at "Justice as Fairness: Greek Debt."