The anti-theft system in early '95 models is fairly simple. When the car is locked using the physical key, the Alarm Control Unit (ACU) cuts off 12v ignition power to the DME. The key activates microswitches in the door lock that momentarily ground the ACU’s "prime" or "unprime" input.

Key points:

System Primed:

- The ACU opens an internal switch, cutting 12v ignition power to the DME.

- The alarm system is armed.

System Unprimed:  

- The ACU closes the internal switch, allowing 12v ignition power to the DME.

- The alarm system is disarmed, and any existing alarms are cleared.

Porsche introduced the Drive Block System in mid '95 to provide enhanced security and remote door locking. System components include: immobilizer ECU (under driver's seat), radio-frequency key FOB, and starter interlock relay, R61.

Drive Block came standard on North American 993's and optionally in other markets. (UK-market 993's use a completely different system). Despite widespread complaints, Drive Block provides robust security by ensuring that the ignition coils and injectors remain inactive until authorized by the immobilizer. This authorization is granted via a digital handshake over the OBDII "Diag K" line only when the immobilizer is disarmed.      

Key points:

System Primed:

• ACU blocks 12v ignition power to DME
• DME prevented from starting engine
• Cranking disabled
• Alarm system armed 
• Immobilizer warning light illuminates for 15 seconds after ignition is turned on.

System Unprimed:

• ACU passes 12v ignition power to DME
• DME authorized to start engine
• Cranking enabled
• Alarm system disarmed 
• Immobilizer warning light illuminates for 2 seconds after key turned on.
• Existing alarms cleared

More detail here.