It is a non-changeable condition that when we play a note or a chord on the piano, we can not do anything else with this note or chord than to wait until it dies out or playing a new note or chord. We can not keep the chord sounding indefinitely, nor can we, after we have played the note change timbre, pitch or other parameters, an option other instruments have available. However, if we do a tremolo, we can get an effect similar to that of holding notes and not let them die out. In a tremolo chord notes are played as fast as possible after each other. Typically, you will make tremolo in octaves or as in 'The Saints' with chord tones.

The Saints Tremolo mark