I'm not sure which video you were talking about but parallel pliers are not spring loaded so you have to supply the force. While you are starting your mains if you have one hand on the pliers holding the string it will be hard to start your mains. Maybe if you included the link to the video your were talking about it would make more sense.
A starting clamp is spring loaded so it is like a third hand holding your string. All Yusuki was trying to do is apply some back pressure on the anchor clamp while tensioning the the first main.
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