Step 1: Select the magnifier icon from the left menu. The icon will turn gray meaning the tool is activated.
Step 2: Select the Record button to start recording. When you are in a situation, where you would like to zoom in a certain part of the slide, move the magnifier to that point and single-click on the selected point with the left mouse button. The content zooms in. Click again to zoom out.
When you finish recording click the Stop and the Save button. All zooming in/out within the presentation will be synchronized with your voice.