Voiding an invoice in Core is just like deleting it, except that Core keeps a record of it. The time and expenses associated with it get released for billing just like they do on deleting an invoice. Check the Core Help for details.
Note: You should not use the void option to write off bad debts in the Invoices screen; you should rather use the Write-Off payment method for that in the Payments screen. Check out this article for details: Writing off invoices that won't be paid.