According to the manual...
For Delay...
Mix Location Pre/Post
Determines whether the Mix control is positioned at the input (Pre) or the output (Post) of the delay. This option does not change the signal immediately, but it becomes relevant when the Mix control is morphed by a switch or pedal.
In the "Post” position, a change of the Mix control changes the output level of the delay, and thus will have an immediate impact to the delay tail when modified. In the "Pre” position, the delay tail will be unaffected by fast changes to Mix, which will control only the delay input level
For Reverb...
Mix Location Pre/Post
Determines whether the Mix control is positioned at the input (Pre) or the output (Post) of the reverb. This option does not change the signal immediately, but it becomes relevant when the Mix control is morphed by a switch or pedal.
In the "Post” position, a change of the Mix control changes the output level of the reverb, and thus will have an immediate impact to the reverb decay when modified. In the "Pre” position, the reverb decay will be unaffected by fast changes to Mix, which will control only the reverb input level