H. Neuhaus (►Bibliography) used this military terminology to define the dependencies present in the motor apparatus of the pianist: in this very accurate comparison the “front line” corresponds to the point of contact between the finger and the key.
As for the “rear guard” , this corresponds to everything that makes up the rest of the motor apparatus, that is to say, according to Neuhaus, the parts going from the hand to the shoulder (personally, I prefer to say… going to the feet firmly planted on the floor).
Besides, the fingers should not be the only ones to be on their guard. This is valid also for every part that makes up the “rear guard” as well as for the pianist himself. All the elements that form pianistic technique must interact in perfect harmony, in order to guarantee the conditions for the fingers’ optimal playing abilities.