What do the speaker and those in attendance expect to experience when "the last Onset" occurs in "I heard a fly buzz when I died"? Plz, answer this question cause I don't get it...

The speaker and those present expect to feel something striking, but that sensation is hindered by the presence of a fly that attracts everyone's attention.

Explanation:

The speaker is portraying the moment of his own burial, where he tries to make it a grand event, as well as those present. As death is a sad moment, the speaker expects everyone to be impacted, to feel strong emotions and to regret the match, but none of this happens because the presence of a fly gets in the way and attracts everyone's attention.

i've read it so how can i ?

the underlined phrase is an example of : understatment