Herbert is using a pulley as a metaphor for the relationship between God and man. A pulley is a simple mechanical device which you use to lift something up by pulling down on it. God lifts us up by withholding the gift of rest and drawing us closer to Himself. The mechanical metaphor neatly encapsulates the reciprocity of the relationship between man and God.
The central conceit of the poem is God's act of creating human beings. God has a glassful of blessings he wishes to pour into us as he puts us together:
So strength first made a way;
Then beauty flowed, then wisdom, honour, pleasure.
The most important blessing, the gift of rest, lies at the bottom of the glass. When God has finished bestowing the other blessings upon us, he hesitates. Perhaps it might not be a good idea, God muses, to include this particular blessing in the making of human beings:
“'For if I should,' said he,
'Bestow this jewel also on my creature,
He would adore my gifts instead of me,
And rest in Nature, not...
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