How does the poet portray the image of the pulley in the poem "The Pulley" by George Herbert?

The pulley is used by Herbert as a metaphor for the relationship between God and the individual Christian believer. A pulley is a simple mechanical device which you use to lift something up by pulling down on it. And God both pulls us down and lifts us up by withholding the gift of rest. One is reminded of St. Augustine's famous maxim that our hearts are restless until they find rest in God.
As with many metaphysical poems, "The Pulley" is based around a conceit, or extended metaphor. In this particular case, the conceit is God creating human beings by mixing various blessings together. Herbert portrays the Almighty as having a glass containing these blessings that he wishes to pour into us as he creates us:

So strength first made a way;
Then beauty flowed, then wisdom, honour, pleasure.

But then God hesitates. He holds back the most important gift, rest, thinking it may not be a good idea to bestow it upon us after all:


“'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 the God of Nature;
So both should losers be.'"



If we were completely at rest, then what need would we have for God? That's why he does not give us natural rest; the only rest we can have is the rest that comes from being lifted up to God. Hence, Herbert uses the pulley metaphor, which perfectly encapsulates the reciprocal relationship between God and humanity.
https://www.poetryfoundation.org/poems/44370/the-pulley