When typing the year and month field, the day may exceeds the max days in current month. The default behavior is as following:
1) If the original day is the last day of the original month, set the day to the last day of the new month.
2) Otherwise, keep the origina day. If the day exceeds last day of the current month, increase the month. (it's done by the standard JavaScript.)
Set this property to true will change to the following behavior:
1) Keep the day if it's valid for the new month.
2) Set the day to the latest day of the new month, if the original day exceeds the last day.

For example, for current date 1997-01-29, set the month to 2.
1) ForceMonth=false, the new date will be 1997-03-01;
2) ForceMonth=true, the new date will be 1997-02-28.

Reference

C1InputDate Class
C1InputDate Members