In a video interview, Zahar says that Arafat gave Hamas the green light to launch terror attacks after the failure of the Camp David summit in 2000 - not after Sharon's visit to the Temple Mount.
Some Hamas leaders thought that this was one of Arafat's tricks to entrap Hamas and give information to Israel about them, but others were convinced when Arafat transferred RPGs to them. The cooperation was given the code name "Ammar al-Mukhtar."
(Zahar also claims to have met Yitzchak Rabin along with a Fatah official during the Oslo process, and that it was a awkward meeting because it was about security cooperation with Israel which Hamas had no interest in.)