The final paragraph of 35 U.S.C. 112 provides a means for the claims to be shortened provided certain conditions are met. Basically, the 6th paragraph states that an element in a claim for a combination doesn't have to recite the structure, material or acts already described in the specification if certain rules are followed. These rules include that the claimed element must be expressed as a means (or step) performing the specified function.

Translated, if you use the term "means" or "step" in the claims, you might not have to include all the details of the claims if the details are discussed elsewhere in the specification. Alternately, you may leave both of these phrases out if the claim is written as a function to be performed and leaves out any other structures, materials or acts which might confuse the matter. An example would be; "a jet driving device so constructed and located on the rotor as to drive the rotor."  

The specification must link the structure, the materials or the acts to the function recited in the means for (or step for) plus function claim limitation. Alternately, it must be clear to a person of ordinary skill in the art from the specification to know what structure, materials or acts perform the function in the means for (or step for) claim limitation.