If you start a tweet with @username, it’s a reply
I've seen so many "official" brand Twitter accounts do this. If your tweet looks like the following one, only the individual will see it, not all your followers.
@pdjohnson is doing a QA session today, use the #AskPaul hashtag to pose a question
An interesting note on the above @reply is that "mutual followers" (people who follow you both) also see the @reply so they may join the conversation.
With a subtle change you can send the tweet to all your followers ...
.@pdjohnson is doing a QA session today, use the #AskPaul hashtag to pose a question
NOTE the . (period) at the start. It's as simple as that.