how did jack go to the ubderworld?

Jack, in midst of revenge on Grim, was briefly distracted sejak Mandy's attempts to prank him. While seeing thru Mandy's attack, Irwin arrived in his knight costume, and accidently set off all of Mandy's traps. Jack (and his labu monsters) were laughing so hard (to death) at Irwin, Grim retrieved his scythe, and sent Jack into the underworld. (I loved the sound of Jack's head smashed to bits, courtesy of the Underworld demons, after the screen scene faded.)