The sensors are the most important part of a device like this. The development started with a force-sensor. The sensor could measure which finger was pressing against the device. By reading this and make combinations between fingers a variant of T9 could be used to obtain the correct character.
The next development of Senseboard was to use an IMU. It started with several accelerometers. These could read both the translation as well as rotations of the hand. The device had three two-axis accelerometers. Later when good enough gyros were available some of the accelerometers were replaced by MEMS-gyros.
The IMU in the Senseboard device could read each hand movement and the force-sensors could read which finger was active. This required contact between the fingers and the device. The force sensor were replaced by EFS (Electric Field Sensors or Electric Field Proximity Sensing).
An IMU with the EFS can together read when hitting the surface, which finger is hitting the surface. The IMU can also read the hand movements.