Duh. Should have read the post first
It's just there to keep the sack from sucking shut when you drain.
On my ballast system, I used to use a piece of 1" OD clear vinyl tubing about 1 foot long, and jam it on a barb fitting that screws on the pump. When you stick it in the sack the 1" tubing is stretched enough around the barb that it seals great at the sack inlet. To remove it, just twist.