Three short lines capture a single moment at the beach: the tide has reached its peak and is about to turn. The speaker compares this turn to a machine shifting gears, which is a surprising choice. The ocean is usually described as wild or timeless, but here it seems almost mechanical, as if some steady engine underneath it is changing speed. The ellipsis after "gears" works like a small pause, the brief stillness before the water begins to pull back.
The final line, "withdrawing sand," is a little odd, because it is the sand that seems to be doing the withdrawing, though really it is the water taking the sand with it as it goes. That blur between the two makes the moment feel quiet and slightly strange.
The speaker doesn't state a strong emotion. Instead, they seem calm and curious, simply watching and noticing. The mood is hushed and observant, with a touch of playful wit in the machine comparison.