Tweak Mode allows changing numeric constants in a sketch in realtime.
When a sketch is being executed in tweak mode, all hard-coded numbers become interactive and can be modified by clicking and dragging to the left or right. When a value change, the PDE will update the running sketch with the new value and the result will be visible immediately. This mode is useful if you want to refine a certain feature/color/behaviour in your sketch, if you want to experiment freely with numbers, or if you try to understand someone else's code. other uses are welcome.
TweakMode is now part of Processing 3.0 and is maintained there, please report issues on the main processing repository. http://github.com/processing/processing
All the details are here: http://galsasson.com/tweakmode/