'lather hog': a short lofted, extremely dense brush (think chubby 1, rooney 1/1 or Emilion) which holds most of the lather 'buried' deep within the knot, and thus releasing it rather sparingly onto your face, or only after some vigorous brushwork. A lather hog has limited 'flow through'.
'flow through': essentially the space between the hairs of your brush - more space makes it easier for water, cream/soap and air to pass through the brush knot, mixing together quicker, and consequently forming a stable lather faster. A brush with a good 'flow through' risks becoming a 'wet mop' when you apply the lather on your cheeks.
:ugeek: