Low-Power library question
It's doing basically this:
clockSource = TCCR2B &amp; (_BV(CS22) | _BV(CS21) | _BV(CS20)); // store prescaler
TCCR2B &amp;= ~(_BV(CS22) | _BV(CS21) | _BV(CS20));             // select &quot;no source&quot; - disable clock source
power_timer2_disable(); // disable whole timer2 in Power Reduction Register

Definition of power_timer2_disable macro is:
#define power_timer2_disable()  (PRR |= (uint8_t)(1 &lt;&lt; PRTIM2))

In short, you have to turn off the clock source before the power gets cut off by PRR register (to avoid unexpected behaviour).

Power Reduction Register
The Power Reduction Register (PRR), provides a method to stop the
clock to individual peripherals to reduce power consumption. The
current state of the peripheral is frozen and the I/O registers can
not be read or written. Resources used by the peripheral when stopping
the clock will remain occupied, hence the peripheral should in most
cases be disabled before stopping the clock. Waking up a module, which
is done by clearing the bit in PRR, puts the module in the same state
as before shutdown. Module shutdown can be used in Idle mode and
Active mode to significantly reduce the overall power consumption. In
all other sleep modes, the clock is already stopped.