The standard Sonoff device has a relay to turn on/off an external device and a button to toggle the state of the relay. Additionally a "Switch" can be configured using the Gpio Command. (See GPIO-Locations for how to connect to these GPIO's.)

Tasmota SwitchMode Explained Video by Dr Zzs

SwitchMode and SwitchTopic are for Switchs only. Not for Buttons!

By default the configuration for the button looks like this:

SwitchMode=0 (Set switch mode to TOGGLE)
SwitchTopic=0 (Disable use of MQTT switch topic) 

SwitchMode=0 means every time the button gets pressed the relay changes its state (on/off).

SwitchTopic=0 means the button press will not send a topic via MQTT. Instead the relay will be controlled directly. Once the relay changes its state the firmware will send an MQTT message with the new state.
e.g.: stat/sonoff01/POWER ON

Important:
The Sonoff-Tasmota firmware does not publish the state of the switch in any way! It only send commands (directly to the corresponding relay or via MQTT to a topic).

You can change the mode for a switch by executing a command in the web console, via HTTP, via MQTT or via serial.

SwitchMode 1

Set switch mode to FOLLOW (0 = Off, 1 = On)

While pressing the button/switch (closing the circuit) the button will send ON (directly or via MQTT depending on SwitchTopic) and OFF when released (open circuit).

SwitchMode 2

Set switch mode to inverted FOLLOW (0 = On, 1 = Off)

The opposite of Mode 1.

You properly want to use SwitchMode=1 when connecting a toggle switch to your Sonoff device. This way the 'software switch' will have the same state as the 'hardware switch'. If the real switch is in the "on" position, the state of the software switch is ON as well.

SwitchMode 3

Set switch mode to PUSHBUTTON (Normally 1, 0 = toggle)

The switch will send a command (directly or via MQTT) when the switch opens the circuit (OFF). When closing the circuit (ON), nothing will happen.

SwitchMode 4

Set switch mode to inverted PUSHBUTTON (Normally 0, 1 = toggle)

The switch will send a command when changing to ON. Nothing happens when changing to OFF.

When connecting a momentary switch (a push button) you want to use SwitchMode=3 or 4.
SwitchMode=4 is the same as the default behavior (SwitchMode=0) for Sonoff devices.

Disable use of MQTT switch topic

No MQTT message will be published on account of the new switch state. The message you see in MQTT is the new state of the relay that gets changed, like explained above. e.g.: stat/sonoff01/POWER ON

Set MQTT switch topic to Topic

When changing the state of the switch (depending on the SwitchMode) an MQTT command gets send via MQTT. This command will have the topic defined as the default topic of the Sonoff device.
e.g.: cmnd/sonoff01/POWER ON (Notice the cmnd instead of the stat at the beginning.)
This command will be received by the device itself and the relay will be set to the defined state. Just like receiving the same command from any other MQTT source.

Set MQTT switch topic (32 chars max)

This will send a MQTT message with a user-defined topic. Whether this message will trigger anything depends on the setup of the user. For example, when setting the topic to sonoff2 this could trigger the relay on the Sonoff device that listens to messages with this specific topic.

To trigger something within your smart home setup (e.g. [openHAB]) you can define the SwitchTopic to sonoff01-switch1 (for example).

This will send a message like this: cmnd/sonoff01-switch1/POWER ON

In openHAB you define an item that subscribes to this topic and sets its state according to the message content.

Switch Sonoff01_Switch1 "Sonoff01 Switch1 [%s]" <switch> { mqtt="<[mosquitto:cmnd/sonoff01-switch1/POWER1:state:default]" }

SwitchTopic=0 controls the relay directly.
SwitchTopic=1 sends a message with the default topic to MQTT. This message will get picked up by the device itself and sets the state of the relay accordingly.
While SwitchTopic=2 sends a command with a custom topic via MQTT. This will not get picked up unless you configure any device to subscribe to this topic.

Examples with Rules