There are 2 instances to Rell’s W. When she is mounted, she gains movement speed, which is reduced if she takes damage. When she is mounted (and when this ability is activated), she will jump of her mount, knocking up enemies and dealing damage. She will then enter an armored form where she reduces her movement speed but gains a shield.
When she is dismounted, she is very slow but gains armor and magic resistance. When this ability is activated, she mounts up gaining movement speed briefly which is increased if she is running towards an enemy champion. Her next attack during the duration throws her target behind her dealing damage.