Whether you spin on a wheel or on a spindle, the object of the tool used is to put twist into fibers to create yarn. On a spindle, each rotation of the shaft inserts one twist. On a wheel driven spindle, each rotation of the spindle inserts one twist. On a bobbin and flyer wheel, each rotation of the flyer inserts one twist.

The direction in which the drive wheel of a bobbin and flyer wheel is turning controls the direction of the twist. This is also true for wheel driven spindles. With spindles, the direction in which the shaft of the spindle turns controls the twist direction of the yarn.

On bobbin and flyer wheels and on wheel driven spindles, when the drive wheel is turning towards the right or clockwise, Z or right twist is being inserted into the yarn. When the drive wheel is turning towards the left or counterclockwise, S or left twist is being inserted into they yarn.

When spinning on a spindle, finger snapping it to the right inserts Z twist, to the left inserts S twist. Using a drop spindle with a thigh roll changes things a bit depending on which hand is used to hold the spindle. If you hold your spindle in your right hand then rolling the spindle shaft up the right thigh gives Z twist, rolling it down the right thigh gives S twist. If you hold your spindle in your left hand, then rolling it down the left thigh gives Z twist and rolling up the left thigh gives S twist.

Normally, singles are spun Z and plying is done S. Yet, it really doesn’t matter as long as twist direction is consistent and singles yarns are plied using the opposite direction of twist. Some novelty yarns do require plying twist to be inserted in the same direction as the twist of the singles to create special effects in the finished yarns so there are some exceptions to this rule.

How the singles yarn feeds off the bobbin for plying can affect the twist in the singles yarns.
This is easily demonstrated with a couple rolls of toilet paper. Pull the TP straight off the roll and it does not twist. Mount the roll of paper onto a vertical kate and pull it off so it’s unwinding from the roll in a clockwise direction and Z twist will be inserted, one twist every time one layer peels off from the roll. Turn the roll over and pull the paper off in a counterclockwise direction and one S twist is inserted for each layer that is unwound from the roll.

So: If plying bobbins are mounted on the kate so the yarn feeds off from the shaft of the bobbin, no twist is being added or subtracted. Any change to twist with this plying feed set up comes from the fibers in the yarn being plied relaxing and twist equalizing and evening out over the length between the plying bobbins and the plying zone. But if the bobbins on the kate are feeding off the ends then twist is either being added or lost.

Bobbins mounted on the kate with the yarn feeding clockwise: Z twist is being added to the yarn.
Bobbins mounted on the kate with the feeding counterclockwise: S twist is being added. S twist removes Z twist so the effect is that twist is being lost.
A good vertical kate has a thread guide which keeps the singles yarns feeding from the shaft of the bobbin. The singles yarns may make a few turns on the way to the plying zone but a simple turn in direction does not affect twist.

To avoid having twist either added or subtracted from your singles when plying, make sure that the bobbins on the kate are feeding from the shafts, not over the flange ends of the bobbins.