Hi kapetan,
If you invest one bitcoin, get hashing power of one Th/s, mining revenue paid in PPLNS mode will be prior to pay your profit, then pay your unpaid capital.
Contract ends only when your one bitcoin is returned in full.
Profit is calculated by the formula: unpaid principal (BTC) * 0.7 (satoshis per BTC per second) * time to find a block (seconds).
So the end result is roughly 22% of profit per year under condition you re-buy your contracts during the entire year. Principal returned in full.