his heart sinking
‘Yes,’ said he, his heart sinking horribly.
or
‘No,’ he thought, his heart sinking.

I usually have an -ing form with a verb 'to be' in some form.
Is this 'his heart sinking' a short form for 'while his heart was sinking' or something else?

Question

his heart sinking
‘Yes,’ said he, his heart sinking horribly.
or
‘No,’ he thought, his heart sinking.

I usually have an -ing form with a verb 'to be' in some form.
Is this 'his heart sinking' a short form for 'while his heart was sinking' or something else?

Answer

No it is not a short form. His heart sinking is a phrase which can stand alone. It gives emphasis to the fact that he is he is becoming more and more unhappy/nervous etc.

Ex:  'That will be $1,000 sir for the ring, is that OK?' 'Yes he said, his heart sinking'. Meaning that the idea that the ring could cost so much is making him feel ill.

Answer

his heart sinking = a gerund phrase

Answer

his heart sinking= he felt sad/disappointed