Horizontal grooves in the forefoot of running shoe outsoles are called flex grooves. As the name implies, flex grooves are designed to promote forefoot flexion across the ball of the foot.
In many cases of forefoot pain, however, this is exactly where you don’t want the shoe to bend. You actually want the shoe to be as inflexible as possible in this area if you have the following forefoot conditions: hallux limitus, sesamoiditis and metatarsalgia.
If you have forefoot pain, deep flex grooves located directly beneath the affected joint will cause an increase in forefoot flexion as well as reduced shock absorption due to a lack of an overlying outsole in the area of a groove.
The shoe on the left side of the image has flex grooves adjacent to where the forefoot bends, which can alleviate forefoot pain. The shoe on the right side of the image has a flex groove where the forefoot bends, leading to more pressure in this area.
If you experience pain in the forefoot, check the flex groove placement in your shoes. Flex grooves in the wrong place can contribute to forefoot pain.
Dr. Jenny Sanders Shoe Blog 
Flex ix not groovy
This is a very timely topic for me, as I’m getting occasional metatarsalgia flare-ups from Mizuno Inspire 8s. I suspect this is caused by these shoe’s forefoot flex controllers, but I may well be wrong.
As a general comment, I would love to see your review of the Inspire 8 and/or Nirvana 8.
And what about nike air pegasus
Nike Air Pegassus 28 horizontal flex grooves are on either side of the ball of the foot so this shouldn’t be a problem.