Here's a quick how-to video that shows you how it works: http://sumac.com/quickbooks-integration/
This all stems from Sumac's Ledger Entries feature. You set up your account codes to match the account codes you have in QuickBooks. Then you set up your ledger entries preferences in Sumac to tell Sumac how it should organize financially significant transactions into your ledger. Here are some how-to videos on setting up Ledger Entries: http://sumac.com/videos-ledger-entries/
Once this is done, as you add transactions (donations, memberships, registrations, etc.) into Sumac, Sumac adds them to the ledger. At the end of the day, week, or month (however often you reconcile with QuickBooks), you can run a ledger report to make sure it all balances, then click the "Export to Accounting" button to send it all to QuickBooks!