I am sharing this piece of knowledge for the folks who might face the same problem.
I am manually upgrading a rooted Motorola Nexus 6 device, for which OTA update is not available due to system ROM modification. A populare guide can be found here or by following Nexus Root Toolkit's wizard.
Android reboots in recovery mode with a "No command" message. NRT and guides say that this is the pre-screen to recovery. I need to enter recovery and enable download from ADB. The problem is that when I do the PWR+Volume+ sequence I only get the device rebooted into Android