Using either a tool or your finger, apply a force forward in the direction of the arrow. This will release the latch. Simply let your finger or tool up without releasing pressure on the latch, and the spring system will cause the battery door to rise automagicly.

The best way to do this with your finger is to grip it from behind with your finger nail and pull the latch upwards and forwards. This allows it to easily lift upward as you pull towards yourself.