How to insert a new unit

Using Normal Mode

Move the mouse cursor to be within the borehole field at the depth at which you want the top of the new unit.
While holding down the Alt key, click and drag with the left mouse button down until you reach the position at which you want the bottom of the new unit.

Using Edit Modes

Enter Insert Mode (use the F6 shortcut key, select Insert under the Tools menu, or click the Insert Mode toolbar icon ).
Move the mouse cursor to be within the borehole field at the depth at which you want top of the new unit.
Click and drag with the left mouse button down until you reach the position at which you want the bottom of the new unit.

Notes

When inserting a unit, the top and bottom depths of the unit are limited by the current Depth Tolerance - at small display scales (zoomed into the borehole) or with large depth tolerances the position of the boundaries may be noticeably different from the position of the mouse cursor. You can watch the current depth shown at the bottom of each borehole field, or turn on display of the Ruler to get a preview of what the actual depth of the new boundary will be.