The d t (calc-truncate-stack) command moves the `.'
line that marks the top-of-stack up or down in the Calculator buffer.
The number right above that line is considered to the be at the top of
the stack. Any numbers below that line are "hidden" from all stack
operations. This is similar to the Emacs "narrowing" feature, except
that the values below the `.' are visible, just temporarily
frozen. This feature allows you to keep several independent calculations
running at once in different parts of the stack, or to apply a certain
command to an element buried deep in the stack.
Pressing d t by itself moves the `.' to the line the cursor is on. Thus, this line and all those below it become hidden. To un-hide these lines, move down to the end of the buffer and press d t. With a positive numeric prefix argument n, d t hides the bottom n values in the buffer. With a negative argument, it hides all but the top n values. With an argument of zero, it hides zero values, i.e., moves the `.' all the way down to the bottom.
The d [ (calc-truncate-up) and d ]
(calc-truncate-down) commands move the `.' up or down one
line at a time (or several lines with a prefix argument).