Answer
The area of ice under the wire feels a great pressure, which lowers the freezing point and causes the ice in only that place to melt. The water above the wire is no longer subject to this pressure and refreezes.
Work Step by Step
From Figure 13-22, we see that the freezing point of water decreases at higher pressure.
The thin, weighted wire exerts a large pressure on the ice, lowering the freezing point to below the usual $0^{\circ}C$, and thus the ice directly under the wire will melt.
The wire moves through the new liquid, farther into the block. The water above the wire refreezes, since the excess pressure on it has been removed.
In this way, the moving wire passes through the ice block and yet leaves a solid block of ice above/behind it.
