I just come across the following PDF page in Internet:
http://www.espoma.com/pdf/garden/GI_4.pdf

It says that we should deep water the lawn. But it also says this:
"During the summer, much of the root system deteriorates.
Therefore, summer waterings should be light and frequent."

This is new to me. I am under the impression that we should always
water the lawn deep, not "light and frequent".

Can someone help me to understand that suggestion?

Not sure there's much to understand -- it's completely contrary to all
convential wisdom on the subject and in short it just doesn't make sense.
Perhaps in particularly hot climates the root system dies off somewhat, but
that does not negate the value of continuing to water deeply -- deep
watering serves the same benefit regardless.

James