What?
Zoom Down is the same as the Zoom Window feature found in many CAD systems – it simply zooms so that the rectangle that you have dragged out on the screen becomes the next view that you would see.

This means that you see lesser of your of your project but now you see the portion in view bigger

How?
Click on the 1st button on the right vertical view bar to switch the mouse into Zoom down mode. Then dragging a rectangle with the mouse will result in that portion of the site getting magnified.

Zoom down is actually available almost always. Here is a simple trick:
For Zooming down, simply press Ctrl when dragging mouse to create a rectangle. That portion of the site will get magnified.

Watch out!!
Under no circumstances will the architect be allowed to shift out of your view. So just in case if the new view that you now be shifting to does not have the architect located there then you will find that the architect is forcefully taken out from its previous location and located to the centre of the new view

This would obviously mean that the earlier location of the architect – which you may have located very carefully – would get lost!