Andrew Leach is right in saying "is emptied" is better than "empties" here.

However, for your particular example, I'd suggest:

"During the operation field a is cleared."

When relating to GUIs, it's a much more common way of expressing that the content has been removed from an input field.

Andrew Leach is right in saying is emptied is better than empties here.

However, for your particular example, I’d suggest:

During the operation field a is cleared.

When relating to GUIs, it’s a much more common way of expressing that the content has been removed from an input field.