How to handle torpor?
Take your finger off the trigger. Take your foot off the accelerator. 

Just sit but with a quiet still silent mind.

Don't attempt to watch the breathing. 

The Buddha did not teach to watch breathing. 

The Buddha taught to abandon craving. 

Every impulse to watch breathing is craving. When this impulse arises, abandon it, realise it, drop it, let it go. 

When you stop trying or craving to watch breath and have a silent mind, Anapanasati (which means 'mindfulness with breathing' rather than 'mindfulness of breathing') can naturally take its natural course. 'Mindfulness' means 'to remember to keep the mind' free from craving. 

When the mind stops trying to watch breathing, the mind will be free from the thought of 'trying'. Not trying actually makes the silent mind more clear, more sensitive. 

Do we need to make an effort to hear a sound, to smell a smell or to see an object? if not, why does an effort need to be made to know/feel the breathing, when the mind is naturally silent?