Cast Sensei's Divining Top by paying 0 due to Cloud Key.
When Sensei's Divining Top enters, Skyswimmer Koi triggers.
Holding priority, activate Sensei's Divining Top by tapping it, causing you to draw a card and put Sensei's Divining Top on top of your library.
Resolve the Skyswimmer Koi trigger, causing you to draw Sensei's Divining Top from your library, and then discard a card other than Sensei's Divining Top.
Repeat
This clever loop turns the unassuming Sensei's Divining Top into an endless engine of card selection and churn. With Cloud Key reducing the Top's mana cost down to zero, you can recast it for free as many times as you like. Every single time that artifact enters the battlefield, Skyswimmer Koi triggers to let you draw and discard, which seamlessly synergizes with the Top's own activated ability to cycle right back into your hand.