Many IoT devices are battery-powered, and to conserve battery, enter sleep modes as much as they can. They usually get out of sleep in either of two cases (or both):
- At fixed intervals, like every 6 hours, or every day for instance
- When they receive an external signal from a sensor
When they wake up, they connect to a server, send their data, may check for data from the server (configuration updates...), and then go back to sleep.
The issue is that if you want to talk to a device immediately, it won't be possible, you'll have to wait for the next wake-up connection.
"Shoulder tap" is a way to remotely wake up the device. The idea is that most of the device is off, and only a low-level cellular connection is maintained, in some low-power mode. Then, whenever the server wants to talk to the device, it sends an SMS, and the modem will wake up the device to do whatever is requested.
This requires the device to have a cellular modem, which is capable of getting into low-power modes but still receive SMS messages, that the modem is able to wake up the device. Maintaining the cellular connexion in a state that allows reception of SMS will most likely increase the power draw quite a bit compared to the usual deep sleep modes, so the benefit comes at a cost.
The exact details vary a lot with the type of device, what kind of battery it can count on, how often and for how long it would be awake or sleeping, the sleep modes in use, the type of modem, the kind of cellular connection, and so on.
What is your actual use case, and what is the actual problem you are trying to solve?