A half-keyboard is a specially designed and programmed keyboard used in limited space situations or when the typist needs a hand free to answer the phone, hold documents, etc.
It usually consists of only the left-hand half of a normal keyboard, but is coded in such a way that if a particular key is held down (usually the spacebar), it "jumps" to being the right half of the keyboard, allowing a person to type with only one hand.
It is said to be quick to learn, because our bodies can easily replicate one motion on one side to the other side, and almost as fast as a normal keyboard.
References
Half-keyboard Wikipedia(Text) CC BY-SA