IF the secondary/focuser axis is perfectly aligned, and IF the primary is perfectly aligned with a Cheshire (reflective circle with dark center), THEN the return beam of a perfectly collimated laser can be used to confirm the alignment of a primary mirror.
But, the barlowed laser technique is more accurate for aligning the primary, and it doesn't even matter if the laser is collimated.
Here is an explanation:
http://www.cameraconcepts.com/barlow...ollimation.pdf