key-up could give you all sorts of interesting information: you could detect "false zeros" where there are extremely rapid up-down sequences suggestive of motor function disorders in the user (Parkinsonism, etc). You could test the forearm extensors. You could test motor units to exhaustion (hold key until press fails, the "keyup" event). We haven't even gotten to the spinal cord yet, so I'm pretty sure the frontal cortex could be engaged in all sorts of interesting 3rd order ways.