Inside your mechanical watch is a fine-tuned machine called the SII-NH35A made by Seiko.

1. MOVEMENT DIMENSIONS
· Outside diameter ψ27.40 mm
· Casing diameter ψ29.36 mm (with dial holding spacer)
· Total height 5.32 mm

2. SPECIFICATIONS
· Winding mechanism Manual winding
Automatic winding with ball bearing
Both winding with one way clutch
· Additional function Date display with quick date correction, second hand reset
· Jewels 24 jewels
· Accuracy -20 ~ +40 seconds per day (23º± 2ºC)
· Shock resistance Shock-absorber device for balance staff
· Vibration frequency 6 vibrations per second (21,600 vibrations per hour)
· Duration time More than 41 hours
· Regulation hands unbalance Hour : Less than 1.50 µN·m ( 150 mg·mm )
Minute : Less than 1.25 µN·m ( 125 mg·mm )
Second : Less than 0.20 µN·m ( 20 mg·mm )

Seiko NH35A top view

Seiko NH35A profile view

Want to take a peek inside an automatic movement? Check this out.

You also need to see this sweet interactive tear down of a mechanical movement over at Hodinkee that explains the critical gears and parts.
The mechanism that makes a movement an automatic is an asymmetrically-weighted rotor that spins the center wheel on ball bearings as you move.

Hodinkee Watch Movement Interactive