Beyond all that, it's also just awesome to watch every enter the battlefield effect on your side of the table trigger simultaneously
The conditions are that you need a chunky creature and that youre hitting a smaller one, but thats not hard for Gruul particularly with so many cheap and pivotal creatures being deployed to curve out
There are two phases to the game against Mono-Red: dont die and kill them. Your goal is to sideboard into all threats that are good at starting off as dont die and rapidly turn into kill them. Planeswalkers dont do that, and Jadelight Ranger usually just gets sent packing by Abrade or Ahn-Crop Crasher
With those conditions met you put the untap spell on the stack, tap Stella to copy it targeting herself, let the copy resolve (untapping her), then use her ability again on the original spell to reset the loop
As the game evolved, the concept of Affinity emerged as a unique mechanic in the "Mirrodin" set, released in 2003