This is a breakout board for our . The four spindles on the trackball have a tiny circular magnet the end; each of these are paired with an SMD hall effect sensor, which are used to measure up, down, left and right movements of the trackball. An SMD momentary switch is placed under the trackball to give you a select switch. The BTN line will be pulled low when the switch is pressed. Project description, more information and ordering can be found at https://www.sparkfun.com/products/9320 Hall sensor datasheet: http://www.sparkfun.com/datasheets/Components/SPC00019AEB.pdf Blackberry Teardown: http://www.freescale.com/files/wireless_comm/doc/reports_presentations/eet_teardown_bbpearl_jan07.pdf Inside the Blackberry (BOM): http://www2.electronicproducts.com/Research_In_Motion_-_BlackBerry_Pearl_8100-whatsinside_text-12.aspx