How to go into an extended-trot?

So, normally when my mare trots its not so smooth, but once she did an extended trot in the saddle and it was amazing! It was unbelievebly smooth and really fast. Normally she'll do it for about 3 seconds before turning into a lope. How can I maintain that or get her into that? I have honestly no clue how I got her into an extended-trot.