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 section 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 M-# u. See section Embedded Mode.