LAST EDITED ON 04-27-10 AT 02:42 PM (EST)
They knew from Amanda that Parvati had an idol. The plan was to get Parvati to play the idol (hence Amanda's little visit) while they voted for someone else. That way the idol play wouldn't nullify the Heroes' votes.
But they needed to keep RussHell in the dark, so they told RussHell they were voting for Sandra. Thus they figured that if Parvati decided to give the idol to someone else to play, she'd likely give it to Sandra (having been informed by RussHell), not Jerri. It was a way of winnowing down their prospects for who wouldn't play the idol. They voted for Jerri thinking that either Parvati would play it or Sandra would -- either way, JT would be safe (at least until the revote) and the idol would be gone.
I still think they should have just voted for RussHell.
(Edited for fine tuning.)
A tribe glows in Brooklyn