This option is selected per default, the moment you select the Coding Mode Lexical. This ensures that sequential Events automatically start on the next video image, preventing tiny overlaps of a single frame.
Should you feel that your Events always start at the wrong frame, feel free to disable it.
As most individual coding options, you find this option in the Observation settings dialog:
▪To open this dialog, click Start - Observation - Observation settings 
.
The following dialog appears:
▪Click OK to confirm your settings.