Here’s a sign explaining the function of dual-flush toilets common in the U.S. It’s a water-conserving mechanism which is simple enough: one pulls the handle upward for a light flush and pushes it down for a full 6-liter-or-so flush.
Here we have a European alternative – a toilet that allows analog control of the flush volume. The user pushes on the right portion to flush. Note the inscription that reads “stop” on the left side of the button – the user can press that at any time to stop the flush.
Here’s the control in mid-flush. In this stage, it reveals an affordance to press the “stop” button, but if the user does nothing, it performs a full flush.
Simple, but it’s a seamless integration of the potential for more control with a graceful default in the absence of further user input.