Nitro in Asphalt 8 has 2 modes. You can either increase intensity of NOS by 3 successive taps slowly or just hit the PERFECT NITRO! It’s a new feature introduced in asphalt 8. When you hit NOS once, a red part appears in the NOS bar. If timing is perfect n screen is tapped while marker is in red region, you will enter perfect nitro mode which is the max power it can offer :)