Sleep and wakeup

Mode with lowest power consumption is power down mode.  The MCU can be woken by e.g. a pin change interrupt.

At wakeup, the interrupt handler is executed, then the code continues where it left of before sleeping.

Pull-ups

Pull-up settings remain valid during power down mode.  So you can leave out many pull-up resistors.

Software reset

The software reset is executed by first enabling the watchdog timer with minimal time interval, followed by running an endless loop.  The first thing the setup() function should do is disable the watchdog again.

Peripherals

Internal ATTiny peripherals can be powered down to limit current consumption.  Don't forget to re-enable them after wake-up or your code won't run correctly.