Yes, that’s true. You can boot into any recovery of your choice on your Android phone by following this guide. No need to flash and replace the stock bootloader. Though I have tested this method on Nexus 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.
Prerequisites
- An unlocked bootloader – It is just a command away on Nexus devices. Google will give you very useful links depending on your device type.
- ADB with Fastboot – Check out this link: http://www.jailbreakeye.com/android/install-adb-fastboot
- A custom recovery image – TWRP or CWM (check the Download recovery or Download touch recovery columns)
Guide
- Copy the downloaded recovery image (.img file) to the folder where you have placed your ADB and Fastboot.
- Launch terminal (or command prompt on Windows).
- Navigate to the directory where you have placed the ADB, Fastboot and .img file together.
- Shut down your phone and reboot into boot loader. For nexus devices, simply hold volume down + power to start up the phone.
- Connect your phone to your computer.
- On terminal (or command prompt) window, write fastboot boot <img file name>, e.g. fastboot boot twrp.img.
- Your phone will restart and will boot into the recovery mode.
That’s it. As simple as it can get!
Tech 'O Mania 
With this method, is it possible to switch between ASOP and TWRP recoveries, or at a minimum, still receive my service provider OTA updates?
Yes. Since this method doesn’t override your stock recovery, your phone will continue to receive OTA updates from your service provider.
Thank you.