Well ACTUALLY... Yes if your battery is LOW, the starter will turn slowerly... however if your battery is more than low... like on its way out, what actually happens is... Click Click Click. Let me explain.
The starter has a Solenoid that trips the motor. In order for the solenoid to engage, it needs X about of voltage. So - you turn your key to start, the solenoid trips, The starter engages (click), the motor causes a voltage drop, which causes the solenoid to trip off - (click), as soon as the motor turns off, the voltage raises, and since only 0.25 seconds have gone by, and your key is still turned to start, the solenoid trips ON again (click).
Continue this cycle, and you get the click click click click sound.
Believe it or not, most dead starters are because of dead solenoids. True dead starter MOTORS - you will get a click and nothing else. That is why the hammer trick works. It is normally a stuck or dying solenoid. If you whack the starter, the vibrations causes the solenoid to jiggle free or loose.