Hot code swapping is a unique and powerful feature of Erlang that allows you to update code in a running system without stopping or interrupting its operation. It enables seamless and dynamic upgrades of live systems, providing high availability, fault tolerance, and continuous operation.

In Erlang, the concept of hot code swapping involves replacing the code of a module in a running Erlang system with a new version while the system continues to execute. Here's how it works:

1. Code Replacement : When a new version of a module is ready, the updated module is compiled into bytecode, just like any other Erlang module. The new bytecode represents the updated functionality or bug fixes.

2. Compatibility : To perform a hot code swap, the new version of the module must be backward-compatible with the existing state and the messages being processed. The new code should be able to handle the messages and state that were created by the old version of the code.

3. Loading the New Version : The new bytecode is loaded into the running Erlang system. The Erlang virtual machine (BEAM) supports loading and unloading of code during runtime. The updated module is typically loaded using functions like `code:load_file/1` or `code:purge/1`.

4. Process Migration : The Erlang runtime migrates the processes running the old version of the module to the new version. The state of the processes is transparently migrated, ensuring that they continue execution with the updated code. The migration is performed process by process to avoid disruptions in the system.

5. Code Swapping : Once the migration of processes is complete, the Erlang runtime performs the actual code swap. It updates the references to the old module with the new module, making the new code active for all subsequent calls and message handling. The significance of hot code swapping in Erlang is :

1. High Availability : Hot code swapping enables continuous operation of Erlang systems with minimal downtime. It allows critical systems to be updated or fixed without interrupting the ongoing processing of messages and requests. This results in high availability and improved system uptime.

2. Fault Tolerance : The ability to dynamically replace code without stopping the system enhances fault tolerance. If a bug or error is detected in the running system, a fixed version of the module can be swapped in without affecting the rest of the system. This helps in isolating and recovering from errors without impacting the overall system stability.

3. Zero-Downtime Upgrades : Hot code swapping facilitates zero-downtime upgrades of Erlang systems. System upgrades can be performed on a live system, ensuring that new features or bug fixes are immediately available without requiring a system restart or disruption to ongoing operations.

4. Continuous Deployment : With hot code swapping, new code versions can be deployed rapidly and frequently. This promotes agile development and continuous deployment practices, allowing developers to iterate and improve the system in a more streamlined manner.

5. Experimental Features and A/B Testing : Hot code swapping allows for the safe and controlled introduction of experimental features or alternative implementations. Different versions of a module can be swapped in and out to test different approaches or perform A/B testing, allowing developers to evaluate and compare the impact of different code versions.

Hot code swapping is a powerful capability that distinguishes Erlang from many other programming languages. It enables developers to update and improve running systems without downtime, supporting high availability, fault tolerance, and continuous operation of Erlang applications.