Yes, that’s true. You can boot into any recovery of your choice on your Android phone by following this guide and that too is without touching your stock recovery. No need to flash and replace the stock bootloader/Recovery. 
Though I have tested this method on Nexus and OnePlus devices only but I am pretty sure that it’ll work on other phones as well. Custom recoveries give you a lot of options for performing some important tasks, like backup and restore your phone and wiping out caches but there if you replace stock recovery you loose the ability to install OTA updates, But with this method you can perform all actions without stock recovery.

Prerequisites:
1. An unlocked bootloader – It is just a command away. As process to unlock bootloader might be different for your device. Google or XDA will give you very useful links depending on your device type.
2. Necessary Drivers:  ADB with Fastboot and your device specefic drivers.
(Note: if you are using mediatek device then also install mtk vcom drivers)
3. A custom recovery image – TWRP or CWM image specifically made for your device.

Guide:
1. Copy the downloaded recovery image (.img file) to the folder where you have placed your ADB and Fastboot(best place to put it is C:/ drive).
2. Navigate to the folder where you have placed the ADB, Fastboot. Hold Shift key and right click anywhere inside the window and look for option "open command window here". 
3. Shut down your phone and reboot into bootloader.
4. Connect your phone to your computer.
5. On terminal (or command prompt) window, 
     a. write "fastboot devices", and it should show your device.
     b. then write fastboot boot <img file name>, e.g. fastboot boot twrp.img.
6. Done! you will boot into the recovery 
7. And after flashing and everything when you boot to system and do a reboot the recovery will be gone i.e you will be reverted back to the previous recovery you had.

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