The evaluates-to operator, ‘=>’, has the special property that any ‘=>’ formulas on the stack are recomputed whenever variable values or mode settings that might affect them are changed. See The Evaluates-To Operator.
The m C (calc-auto-recompute) command turns this
automatic recomputation on and off. If you turn it off, Calc will
not update ‘=>’ operators on the stack (nor those in the
attached Embedded mode buffer, if there is one). They will not
be updated unless you explicitly do so by pressing = or until
you press m C to turn recomputation back on. (While automatic
recomputation is off, you can think of m C m C as a command
to update all ‘=>’ operators while leaving recomputation off.)
To update ‘=>’ operators in an Embedded buffer while automatic recomputation is off, use C-x * u. See Embedded Mode.