One way of doing this (sailing on the footstraps) is to think that you want to push the board with your foot.
When you´re not planing the loads at play are not big and then stand upright is enough to keep things balanced.
But once the wind kicks in you will have to use your body to balance all the different forces ( in direction and strenght ).
So it's like the sail pulls you and in reaction you push the board using your body anchored in the footstraps and hooked on the boom line.
You can do that not hooked nor in the footstraps but you won't last much
As the board increases speed to go on a plane you move towards the footstraps ( out and backward ).
FT 148 has many option to insert the foot straps. Start with them at the more inboard and forward positions. As you feel more confortable start to move the straps more out and backward.
After some time you´ll go to the straps quite quick and will hook when you feel its best for you.
Hope this helps.