Sentence:  The <WSD>helicity</WSD> associated with the writhing number is obtained by observing that a torus with twist number ±1 may be distorted into a figure-8 configuration which appears untwisted....Note that at each step the total <WSD>helicity</WSD>, consisting of the sum of twist, kink, and link <WSD>helicities</WSD>, is conserved. 
Meaning of the word marked with <WSD> tags:  Any of certain measures of the extent to which vortex lines (in fluid flow) or field lines (in a magnetic or electromagnetic field) kink and twist and/or link and coil around one another.