Paste Overwrite command overwrites the audio stream stored in clipboard to the point of the cursor of the target stream. 

Regarding sampling rate the source and target audio streams have to have the same sampling rate. If this is not the case the source audio stream sampling rate will be adjusted to the target sampling rate causing an undesirable effect. 

If the source audio stream is mono and the target is stereo the overwrite operation will take place in both channels of the target audio stream. If the source is stereo and the target is mono the source audio steam paste mixed will be converted to mono and then overwrite operation will take place in target audio stream. If both source and target with the same number of channels the overwrite operation will be executed from channel to channel 

The same result could be achieved with drag and drop operations. Pressing Shift+Control button+Right mouse on the selected area, the mouse pointer changes to “drop pointer”. Once you drag it to the point of the target audio stream (holding shift+Control down) the mouse pointer changes to “Paste Overwrite” style showing that you can do the action. 

Releasing the mouse it will overwrite the selected stream at that point. 

In picture below you can see the effect of overwrite action of the selected region from the first audio stream to the “Untitled 2’” stream. The second stream “Untitled 1” presents the “untitled 2” stream before overwrite action. 

As you can see the length of the file after overwrite action remain as it was before.