Qualcomm EDL mode
The Qualcomm Emergency Download mode, commonly known as Qualcomm EDL mode and officially known as Qualcomm HS-USB QD-Loader 9008 is a feature implemented in the boot ROM of a system on a chip by Qualcomm which can be used to recover bricked smartphones. On Google's Pixel 3, the feature was accidentally shown to users after the phone was bricked. Device support 

For a device to support EDL it must be using Qualcomm hardware. The most widespread SoC from Qualcomm is the Snapdragon. Access

ADB 
The Android Debug Bridge can be utilized to get access to EDL mode, with the command adb reboot edl. Windows 
The Qualcomm Product Support Tool (QPST) is normally used internally by service center executives for low-level firmware flashing to revive Android devices from a hard-brick or to fix persistent software issues. To flash the firmware, the tool communicates with supported devices via EDL. The QPST has not been officially released by Qualcomm. Linux 
Qualcomm Download (QDL) is a tool to communicate with Qualcomm System On a Chip bootroms to install or execute code. The source code is maintained by Bjorn Andersson aka andersson. Test points 
Qualcomm implemented motherboards always include a test point. These can vary from phone models. Generally, test points can be a pair of contacts, and in some devices can be separated. EDL can be accessed by opening the back of the phone, finding sources of the test points that depends on the model, and by using a pair of metal tweezers, which works in most Qualcomm based devices, to short the connectors, and boot the phone into EDL that can be later taken actions with the use of the necessary tools. EDL Deep Flash Cable 

Qualcomm implemented motherboards, with the presence of EDL, can be booted to EDL via the use of a EDL Deep Flash Cable. This specific cable has a general appearance of a button present in the cable. The button can be represented as a switch, to be able to make the phone boot into EDL mode. With the use of the cable, in most devices and cases, it will not be necessary to use the test points. The cable also works on hard-bricked devices to boot them into EDL mode. References 

Qualcomm