Kind of correct.
Target knobs are are not in addition to the regular knobs but replace them. What you do is zero your rifle normally. Once zeroed you then loosen the tention screw that will let you move the knob w/o changeing zero. "Zero" the target knob and tighten that baby back up.
Now, this is were the Target Knobs come into their own. They are zeroed with the rifle zero. Say your out shooting on a windy day. Your shots are off 1" @ 100 yards. If you scope has .25" adjustments you just click in four of them by turning the target knob and you are now zeroed for that day.
The same applies for range adjustments. If you know your bullets trijectory and your scope adjustments you can just click in the required adjustments and be on target. When done just click back to your normal zero.
Target Knobs make it much handier to make adjustments lik jcoyoung said.
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Schmit
GySgt, USMC(Ret)
NRA Life, Lodge 1201-UOSSS
"Si vis Pacem Para Bellum"