This is the explanation to the new @replies policy for Twitter:
We’ve updated the Notices section of Settings to better reflect how folks are using Twitter regarding replies. Based on usage patterns and feedback, we’ve learned most people want to see when someone they follow replies to another person they follow—it’s a good way to stay in the loop. However, receiving one-sided fragments via replies sent to folks you don’t follow in your timeline is undesirable. Today’s update removes this undesirable and confusing option.
Source: Twitter Blog
Basically when someone you follow @replies to someone you do not follow you will not see that @reply.
This strikes me as bonkers. Most of us have seen someone @reply to a person we are not following. This has then sparked our interest we have gone off and looked at both sides of the conversation. We then get involved by adding our opinion, then we all start following each other. There is a description for that, oops wait, wait its on the tip of my tongue, oh yes Social Networking 
Twitter to me these days seems like more of a social experiment than a social network. Every time something like this happens I imagine Twitter founding fathers whacked out on JD staggering round saying stuff like “Let’s see if the Twitter users will put up with this shit” then they go off and implement a very unsocial function.
I don’t know what Twitter are trying to achieve. I wish they would explain clearer in their blogs what they are doing.
< insert here picture of a blue bird a gun and a very injured foot >
The work around: If you are not a fan of the current @reply system all you have to do is add a character before any @reply you do. Example “H @ernmander”
It's quite inexplicable isn't it. If they have any sense I have the feeling that they will have reset it by the weekend. At least set this as the default and give users the option of re-enabling the old style replies via their settings. Maybe a bit of Bernie Ecclestone style smoke and mirrors stuff is going on – do something that causes a rumpus and creates lots of press and then suddenly change your mind and start saying the exact opposite a week later…
I don't quite know what to make of it yet. @Biz added an addendum to it saying more changes are coming that will make things better, maybe this is just the first of it. Maybe they're adding threaded @replies or something. I don't know. I can't quite get my head around what the fuss is yet – but I'm sure the fuss is justified
I'm just not quite understanding how I'm affected yet.
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Do not understand why this post has not been picked up on. You sound like a person who uses Twitter. I see these arse links about this @ stuff but yours makes sense
Tara thank you I try to keep my blogs simple. But hey maybe I am not flavour of the month
Thank you for your kind comment 