The word 'atonic' means 'loss of muscular tone or strength to contract'. Atonic postpartum haemorrhage is characterised by excessive bleeding when the uterus is not well contracted after the delivery, and is soft, distended and lacking muscular tone.
More precisely, in atonic PPH, the myometrium (the muscle layer in the wall of the uterus) fails to contract and compress the maternal blood vessels that tear as the placenta pulls away from the wall of the uterus. Most bleeding after birth comes from the place where the placenta was previously attached. If the myometrium fails to constrict strongly, it cannot compress the blood vessels to control the bleeding.