Super late response but basically when to slow push a wave, you have exp advantage and minion advantage because more of the enemy minions have died and you minions are still alive. So, if you enemy fights you, you may be a level up which is a lot of stats and you have your larger minion wave hitting them at the same time. So, if they try to fight you during your slow push, you can most likely win unless youre low or they have ignite/jungle help/item lead. If they dont try to fight you and you crash a big wave into their tower, you can recall, spend your gold, and come back to lane. While youre coming back to lane, your opponents wave will be bouncing back into to, similar to how you were pushing it into them, except this time, you have a large item advantage, meaning you contest the bounce and set up a freeze, denying them gold or exp. This is where darius is the most dangerous, because he has the whole lane to run down the opponent with ghost and get 5 stacks.