Sentence:  A single gradient (part B) usually will suffice, although to obtain maximum purity (e.g., for use as a probe), a <WSD>velocitization</WSD> step (part D) and a second CsCl-PI gradient (part E) are added. The <WSD>velocitization</WSD> differs from equilibrium gradients in that molecules are pelleted at a rate proportional to their size and largeley independent of conformation. 
Meaning of the word marked with <WSD> tags:  (microbiology) The process of subjecting a sample to extreme speeds