A tsunami is coming, and you want to walk over it, so you have to ford it. Think about it – you ford a river, you ford anything that’s water, unless you’re Jesus – you’re not Jesus. You don’t walk across a tsunami, you ford it.
Your ability to ford this tsunami impresses everyone who witnesses it, especially the Shougun (しょう). Now that you've taught him a tsunami can be forded, he's going to start sending his troops through them instead of waiting for them to pass.