Pin tool 
is used to pin/unpin card(s) on a specific position. Pin tool is located in the bottom right corner of the board.
Using pin tool is simple, just click pin tool to select it and then click card(s) you want to pin/unpin.
Pinned cards can't be moved by drag & drop and won't be relocated while generating or improving timetable.
When to use pin tool?
Pin tool is useful when you are satisfied with card(s) position and you want to assure that those cards won't be moved by starting the generator or by improving the existing timetable.
How to pin/unpin faster?
With pin tool selected click on day or period (in timetable header) to pin/unpin the whole day or period or click on a timetable row (first column) to pin/unpin the whole row.
There are also keyboard shortcuts: Shift key affects the whole timetable, Control key affects the row and Alt key affects the current column. You can do the same even without using pin tool, e.g. click on pin tool with Shift key pressed and all timetable cards will be pinned.
To drop tool just click tool once again or click on an empty timetable period or use Esc key.
As always, undo 
and redo 
can be used to revert or to redo the last timetable change.