In Marvell's poem, "To His Coy Mistress," excess used in the imagery present makes it very hard to find understatment.
The author's continued use of hyperbole—extremes, exaggeration— leaves one feeling (as perhaps does the object of his desire) overwhelmed.
In stating that he would, if time allowed, love and adore her for thousands of years indicates the lengths to which he would go in order to give her the attention she deserves. The speaker's purpose with this exaggeration, of course, is to get this woman to come over to his way of thinking.
If there were a line I needed to identify using understatement in this poem, it would be, "The grave's a fine and private place, / But none I think do there embrace."
In the midst of all the excess the speaker describes, the reference to death seems anticlimactic.
The speaker goes to great lengths to persuade this woman to give in to his advances and sleep with him, but when he speaks of he grave, he describes it as a "fine and private place," as one might describe a lovely room where the lovers might meet. He lamely continues, "But none I think do there embrace." This is also weak: as if he were saying, "I don't think they're having much fun in the grave."
Death is the end, the consummate conclusion to life. Should this not be the most powerful, the ultimate, exquisitely persuasive argument to use? If she says no, their lives will move forward with plenty of chances to love again; perhaps just not for them. With death, there are no second chances: no one returns. One would think that when presenting this part of the argument, his word choice would be much stronger, especially as he seems to become more passionate as the poem moves forward.
Andrew Marvell's "To His Coy Mistress" is generally a poem of hyperbole, exaggeration used for effect. The speaker reminds his love that they do not have all the time in the world; but if they did, he would spend a hundred years just to praise her eyes and forehead and thirty thousand for the rest of her. This is hyperbole, along with his mention of the rivers of ancient times (such as the Ganges and the Humber and the reference to the Flood). The first thirty lines are spent in this vein of grand and extravagant poetic imagery; then come two lines of understatement:
The grave's a fine and private place,
But none, I think, do there embrace.
Understatement, in general, is used to minimize the importance or seriousness of something, again for effect. That's true of this image, for the grave is an ultimate and eternal finality though the speaker makes a rather lighthearted joke about it. This understatement is accentuated by its placement in the poem--right after a string of overstatements. What is being minimized is the time his coy mistress has to make up her mind--which is not, probably, very long.