Usually the plugin modifies the packets sent to the client by changing all ores that aren't exposed to a transparent block like air, water, glass, or lava to stone; and then changes the ores back into ores once they are exposed. Another method I've seen is that it changes all unexposed stone blocks into ores. Now I just wish I could get an anti-xray plugin to work on my server... lol
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If I've helped you, click that little green arrow at the bottom-right. | Check out my Minecraft server! If you say "pls" or "plz" because it's shorter than "please", I'll say "no" because it's shorter than "yes".
Sorry if this is 12 years late, but imagine you use xray, right? And you're in a server. You see diamonds on the xray, so you dig to it, but the moment you mine the block in front of the ore, exposing the ore, the diamond ore instantly turns into a copper ore! That is how they prevent xray in servers.