To be more precise, trigger_relay allows you to trigger entities in a specific mode. Normally all triggers trigger using "Toggle", but with trigger_relay you can specify "On" or "Off", not just Toggle, which is useful if you want to make sure you trigger, for example, an env_sprite off in a specific case, regardless of whether previously it was turned on or not. In such a case triggering with "Toggle" will turn it on, but triggering an env_sprite that is already off with "Off" mode will not turn it on.