[quote:22jp7r53]Why must it be different than a run or jump key press?
It doesn't have anything to do with the movement physics. Platform behavior is preset behavior that makes the physics like collisions easier. It is also possible to make your own platform physics using events but, that is quite advanced. That's why it is as behavior. Also changing collision box isn't something that behavior should do.
I would suggest you set Default controls from the behavior to "No". Now you have more control over the movement through events.
[quote:22jp7r53]why can't i have a down key press for a slide or duck?
Of course you can if you make it so.
[quote:22jp7r53]It would omit an extra step needed to add an animation on a DOWN button press.
If you consider ducking as an extra step then how are you gonna make any other interesting stuff happen in the game?
Conclusion. This is not a thing that the platform behavior is supposed to do and if you have questions about it you should ask in how do I section or look from tutorials.
I have one platformer tutorial there that has Mario game style ducking.
https://www.scirra.com/tutorials/753/mario-style-platform-example-with-spriter-animated-player
From events look under. Player -> Player_Controls -> Player_Controls_Common