It talks about thinking of energy in terms of capital – when you’re short on cash you preserve and save it – when you have an abundance of money you invest.
It makes perfect sense to push yourself really hard on days where you have a surplus of energy – run a bit faster, do a few more laps, sets, reps etc. When you have the additional energy invest it in your body.
At the same time there will be days when you are not bursting with energy – it’s on those days that you preserve your energy
Could be because of a good hard workout the previous day, could be the day after a race or other event when you give it your all – or you could be coming down with something.
But as Jessica concludes – “it’s never about not working out” – you still move about, go for a walk, do some light exercise – but without draining your last bit of energy