What does “Tell devices to save data” do?

Tell devices to save data asks the phones and tablets that join your hotspot to treat it as a limited connection, the way they treat a cellular connection. They then hold back large background downloads, such as photo backups and app updates, while they are on your hotspot.

It is useful when the internet you are sharing is a cellular connection, or any connection with a data cap. It is off by default, and it is available on every edition: Hotspot Lite, Hotspot PRO and Hotspot MAX.

What it does, and what it doesn't

iPhone and iPad turn on Low Data Mode for your hotspot's network. Connectify sends this signal in the hotspot's Wi-Fi announcement.
Android phones and tablets treat your hotspot as a metered network. Connectify sends this signal when it gives the device its address.

Connectify Hotspot doesn't limit, slow or block anything itself. Each device decides what to hold back. If you need to stop a device from using the internet at all, use the switches on the Clients screen instead.

Turn it on

Click Settings, then Network.
Under Wireless, turn on Tell devices to save data.
If the hotspot is running, stop it and start it again, then have your devices reconnect to it.

Settings, Network, Wireless: Prefer Wi-Fi Direct on and Tell devices to save data on

Changing this setting doesn't restart a running hotspot. Android devices are told the next time they get their address, but iPhones and iPads are only told when the hotspot starts. That's why it's best to stop and start the hotspot after changing it.

On Hotspot MAX you can also change it from the command line, with connectify_cli lowdata on or connectify_cli lowdata off. Run connectify_cli lowdata on its own to see whether it is on. See Using the Connectify Command Line.

It uses Wi-Fi Direct

The signal for iPhones and iPads can only be sent over Wi-Fi Direct, so while Tell devices to save data is on, Connectify Hotspot uses Wi-Fi Direct for the hotspot even if Prefer Wi-Fi Direct is off.

Connectify only uses the Wi-Fi Direct driver and API on your PC. What it creates is still a regular Wi-Fi hotspot: your devices join it the usual way and don't need to support Wi-Fi Direct. See What is Wi-Fi Direct?

If your PC's Wi-Fi adapter supports only Microsoft Hosted Network, Connectify can't send the signal to iPhones and iPads. Android devices are still told.
Android devices are told only when your hotspot uses Routed network access. In Bridged mode and in Wi-Fi repeater mode, devices get their address from your other router instead of from Connectify.
To use Microsoft Hosted Network on an adapter that supports both, turn off Tell devices to save data as well as Prefer Wi-Fi Direct.

Checking that it works

iPhone and iPad

Go to Settings > Wi-Fi on the iPhone or iPad and tap the i button next to your hotspot's name. Low Data Mode is on.

Android

Android doesn't show clearly in its settings that it is treating a network as metered. The easiest sign is that Google Photos stops backing up and says it is waiting for Wi-Fi, or that the Play Store holds off on automatic app updates.

If an Android device still uses lots of data, check that its own setting for your hotspot's network isn't overriding Connectify. In the network's settings on the device, leave its network usage (or "Metered") option on Detect automatically.

After you turn it off

Android devices go back to normal once they reconnect to the hotspot.

iPhones and iPads remember Low Data Mode for each network instead of checking again every time they join. So after you turn Tell devices to save data off, an iPhone or iPad keeps saving data on your hotspot. To clear it, either:

switch it off yourself: on the iPhone or iPad, go to Settings > Wi-Fi, tap the i button next to your hotspot's name and turn off Low Data Mode, or
forget the network and turn Wi-Fi off and on again: tap the i button next to your hotspot's name, tap Forget This Network, then turn Wi-Fi off and back on, and join the hotspot again. Doing only one of the two isn't enough.