In early 1995 models, the anti-theft system is relatively straightforward: locking the car with the physical key opens a switch inside the Alarm Control Unit (ACU), blocking 12V power to the DME. While this prevents the engine from starting, it's easy to defeat by jumpering the ACU switch.

Porsche introduced the Drive Block System in mid-1995 to enhance security and enable remote door locking. Key components include the immobilizer ECU (located under the driver’s seat), a radio-frequency key FOB, and the R61 starter interlock relay.

Drive Block was standard on all North American 993's and optional in other markets (UK-spec 993s use a different system). While often criticized by owners, Drive Block offers much stronger protection by preventing the ignition coils and fuel injectors from activating unless authorized by the (disarmed) immobilizer. Authorization occurs through a digital handshake over the OBDII “Diag K” line.

Key points:

System Primed:

• ACU blocks 12v ignition power to DME (same as in early 95's)
• 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.