Because it is originated from a convolution of a Gaussian and an exponential, which is allowed to increase until the Gaussian's centroid, it is an asymmetric function. Therefore when the left skew is added to a Gaussian peak, it causes slight increase at the low energy side of the peak.
The fitted parameters for Left skew are the Left Skew Amplitude (relative to the Gaussian's amplitude), and the Left Skew Slope. The width of the left skew is not fitted, but assumed to be always equal to the width of the Gaussian. The picture shows a left skew with Slope=1 and relative Amplitude=0.25. Please note that skew may take part significantly in the total peak area.

LSAmpl:
 Left Skew Amplitude relative to that of Gaussian (may vary from 0.0 to 0.75)
 
LSSlope:
 Left Skew Slope (0.3 – 2.0)
 
Ampl:
 Amplitude of Gaussian
 
Pos:
 Position of the Gaussian centroid
 
Width:
 Gaussian width (Width = FWHM/1.66),
