I'm just curious to know what the programming mechanics behind it are - more & more, when I tweet or RT something, I get a "mentioned you" spam back almost instantly.
This account is not following me, so what technique are they using to detect that my account has just tweeted?
They use the Streaming API in order to get a real-time feed of all Tweets containing certain words, expressions or hashtags (it's like a real-time search), then they can retweet or reply if they want to. If you don't want unauthorized people to see your Tweets, then change your account to Protected in your Account settings; that will only allow people who follow you (and who you accepted - they can't directly follow you, only send you a follow request) to see your tweets - they won't appear in any public search or in the streaming API.
