Unlike traditional futures, perpetual swap does not have a predetermined expiry date. Funding rate plays the role of adjusting perpetual mark and Bit.com index.
When perpetual swap trades above index price (Mark Price > Index Price), long perpetual accounts will need to pay funding fee to short perpetual accounts, so as to incentivize short trades.
When perpetual swap trades below index price (Mark Price < Index Price), short perpetual accounts will need to pay funding fee to long perpetual accounts, so as to incentivize long trades.