This feature is available when Fit window mode is turned off and currently viewing image is scaled so, that it is occupying at least 50% of width or height of the View area. To start scrolling the image, just press left mouse button when mouse cursor is over View area and do not release it (but do not move mouse at least the first second!). The image starts scrolling and then you can move mouse keeping its left mouse button pressed to control speed and direction of the scrolling process. The mouse cursor far from the center of view area the scrolling speed is faster. And a vector from the center of View area to mouse cursor determines a scrolling direction.
Another way is the direct drag, as usual for most of image viewing applications. To do so, press left mouse button on the image and immediately start dragging it with the mouse, keeping left mouse button pressed down. This way is not so good though, still you will not be able to drag the image far enough and to continue dragging it to the same direction you should release mouse button, return mouse cursor approximately to the same position as before starting such drag, and then repeat dragging. And if the image is large enough you will be necessary to do so many times. So I recommend to master the first suggested way.
If you need even faster moving a certain part of the image to the center of view, use left mouse clicks instead: this will center clicked point of the image in the View area and after several such clicks image will be moved far from the starting position. Certainly there is a sense to do so if the image is large enough.
Having feature "Image scroll with mouse drag" we do not need in scroll bars, which could take some space of the window but is not very convenient still you have to be a William Tell to target it with the mouse always.