Puck is a Max MIDI Device that allows you to use one note to output multiple notes that are currently held. The basic idea behind the device is to be able to play chords using one note. The device uses the [mxj] object so you will need Java installed for it to work.
The first parameter is the Trigger Note. This allows you to select the note that is used to trigger currently held notes. By default this is set to C-1. This is quite a low note that you're unlikely to use when playing or creating chords. The 'P' square will be highlighted green when the trigger note is being held.
The second parameter Velocity Percent, allows you to specify how much the Trigger Note's velocity will affect the output notes. When set to 0, Puck outputs the velocities of the held notes, and at 100% the notes will be scaled to a range of 0 - 1.5 times the original notes velocity.
There's also a small green button in the bottom right hand corner which will send a send note off messages for any stuck notes. Here's a little demo of the device, note that Puck will only trigger notes that are currently being played. You can see this demoed at around 1:20:
You can grab the device over at the Max for Live database: