The bristles of a waterbrush are normally just moist or damp, they're not dripping wet (Photo 1). The water seeps gradually and continuously from the water reservoir down into the bristles, keeping them moist.
To get more water in the waterbrush's bristles, you squeeze the water reservoir. (As you can see in Photo 2, this particular waterbrush even tells you exactly where to push.) Basically you move your hand up a little way along the brush handle, then squeeze with your fingers. Though this feels odd at first, you'll soon get used to this action when painting with the brush.
How much additional water is pushed down onto the bristles depends on how hard and long you squeeze the water reservoir. As you can see in photos 3 and 4, the bristles will hold a fair drop of water before it drips off.
Just how moist the bristles are in a waterbrush depends on the brand. With some the water seeps slower than others, so I suggest trying a different brand if the first one you buy doesn't work well for you. Of the waterbrushes I've got, my favorite is Kuretake waterbrush (used for this photos in this article).
