I suppose the term sitting tall really means don't colaspe or tip forwards!
In your downward transition like Fiona says, you will squeeze your butt muscles (make sure you don't over do this as people often do to much and end up looking like they are trying to prevent an accident from occuring

) Play around with it to see just how little you need!
You will also need to make sure you hold your core muscles so you don't colapse though your middle, tipping the pelvis forwards. Keeping the rib cage lifted help you sit up (some people achieve this easier if you think about lift up though your sternum) so you maintain your frame... I liken a bit to dancing .... have you seem dirty dancing
This stops you rounding the shoulders off which also would tip you forwards as you come to halt. The main thing is to remain toned not tense though your muscles, you can't be ridged or this will transfer to the horse hense why when we try to much, we often create tension and it all falls apart! Gulity as charge mi lud!

So maintaining your frame (with out sticking the boobs
out as that just hollows the lower back!) keep your tummy toned, do as fiona says when you ask for the downward transition and hopefully you should start to feel a difference, play around with it and see how little you can get away with and the more you do it the quicker the horse be
HTH a bit!
