1.5 squat is used to strengthen the lower part of the squat. It typically target the VMO and strengthen the stability around the knee joints. I learned this exercise from Charles Poliquin.
Perform a deep squat, go back up half way and stop for 3-5 seconds. Go back down into deep squat before you come all the way up again. That’s one repetition.
If you can’t perform a deep squat stop at the lowest point and paus for 3-5 seconds before you come back up. Try and go lower and lower.
Don’t rush the movement. I don’t understand why people are in such a hurry performing strength exercises. Calm down, control the movement and feel the muscles working.
I don’t waste much time at slow heavy half squats, I believe the upper part of the squat movement is better trained with more explosive movements like power cleans or jump squats. But that’s just me. You do what is best for you. Deep squats on the other hand should be performed slowly and with full control. Strengthening the deeper part of the squat will give you better strength around the knee joints which is beneficial for running and jumping performance.
Remember now, just because some exercises are said to be most beneficial for performance doesn’t mean other exercises are bad or unnecessary. It’s the overall strength that gives you the most bang for your buck, and you can never perform better than your weakest points will allow.