The main aim of muscle firing is to assist in a given task, this might be reaching out to grab a cup or carrying your groceries to the car.
A natural tendency of the body is to expend as little energy as possible on menial tasks unless there is a direct threat.
Your nervous system (spine and brain) have patterns for every movement and through practise these patterns become more efficient and more powerful.
In order to supply the right movement pattern and muscular tension some information is required; What is the reason you are performing this action and what will you get out of it, but also what are your hands and feet doing
Tension over position
First and foremost I would like to share with you that it is not the position of your body and joints that decide which muscles fire, but which muscles are being tensed. Where do you put the pressure.
Muscles generally work in chains and transducts force via fascia (tissue surrounding muscles). If you want engage your big muscles (torso, hip) the focus needs to be on your hands and feet.
Explosive movement versus Controlled movement
''Train your grip and strengthen your feet first before anything''