Hamas has been desperate to do something about the Israeli intelligence network in Gaza. With the help of their Iranian advisors, they came up with a plan. First, they executed a few (real or suspected) Israeli informants earlier this year in order to get everyone's attention. Then, they announced a two month amnesty for Israeli informants. This included keeping secret the names of those who came forward. Over 200 people took advantage of it, and many of these people supplied names of those they suspected were also working for the Israelis. Naturally, the Israelis don't let informants know anyone else in the intel network, except for their controller (who was sometimes another Palestinian) Then several hundred more suspected Israeli spies were arrested. Many of these were innocent, and a lot of them were simply anti-Hamas Fatah members. What the entire process did do was make all Gazans aware of how large the Israeli informant network was. But that's not all. There are lots of Hamas, Fatah and Egyptian informants. Thousands of Gazans make extra money by keeping an eye on their neighbors, and reporting what they see to someone.
The Iranian counter-intelligence advisors also supplied special electronics that enabled Hamas to detect many of the tracking devices Israel has planted in Gaza, on both structures, vehicles and even people. Hamas was aware of the trackers for some time, but lacked the specialized equipment needed to easily find these devices. Iran, with its huge secret police organization and international procurement contacts, could supply this specialized stuff.
All this hurts Israeli counter-terror efforts, which rely on fresh intelligence, and lots of speed in carrying out raids and attacks. The objective is simple; catch the terrorists before they can attack Israelis. A key source of intelligence is an extensive informant network. Israel was able to build and maintain such a network because they have so many police and military intelligence personnel who speak Arabic. A large segment of the Israeli population even looks like Arabs, because they are descendents of the half a million Jews expelled from Arab countries in the late 1940s. Few Palestinians willingly provide information to the Israelis, so the Israelis developed the BBE (Bribes, Blackmail, Extortion) System. This approach sought out Palestinians who could be bribed, blackmailed or extorted to provide information. This provided enough information to identify the key people in the Palestinian terrorist organizations. The Israelis then staged raids or attacks to arrest or kill these key people. It wasn't just the organizers and leaders they were after, but the "technicians" who could build bombs. The Palestinians called these guys "engineers," and they were a talented bunch. You can note their absence by the increasing number of accidental explosions in Gaza, as less expert bomb builders made fatal mistakes. The lack of leadership meant that more and more suicide (and other type) attacks failed. It's not easy staging a suicide attack. The actual suicide bomber is only one person in a team of up to a dozen people who make the bomb, scout the target area and figure out how to get the bomber past Israeli security.
Israel also has a world class electronic monitoring system. This makes it difficult for the Palestinians to communicate. If they use a any kind of telephone or radio, they risk being overheard. If they speak in code, the Israelis have databases of information on codes used before, and codebreaking software, and analysts, to quickly break new codes. All information is stored in databases and can be quickly retrieved. The planning process for new raids has been automated, and the troops, police or commandoes can be briefed and on their way in hours. This means that if a terrorist operation is detected at any point, the Israelis can usually disrupt it.
Hamas wants to weaken this Israeli intelligence advantage, and their Iranian friends are helping. But the Israelis respond well to pressure and adversity, and are expected to maintain an effective informant network in Gaza.