Re: MordSith
According to the original series, the primary ability of the Confessors is additive, but only during the right conditions the Confessor can release the opposite side (subtractive) known as the Con Dar. Kaylan discovered the Con Dar (not positive) in the second book, before the Silph was discovered. Since she had used (activated) the subtractive side, she was able to ride the Silph. She does not gain proper control over it in the fifth book where I have gotten upto.
As a side note, if the MordSith is either touched by or captures the Confessor's power, it will within minutes drive the MordSith insane, not kill her outright.