Simple.. Their screwing up strides. We all know every step is gonna be different. An then you place this mini hurdles with specific spacing, lets say 2ft or 3ft apart for example..
But what if your tight that day, you step wrong, you stumble, etc.
Your gonna overstride to reach it or your gonna break them or even worse.. 1 of my guys said when he was out at Altis he saw guys stumble & break plenty of them. He even used them & swears they slowed him down cause he's used to a set pattern, that was too short for him, then it was too long..
& when I pointed this out everyone got their panties in a bunch.. I got like 1000 reasons why they should use them.
I gave those up top, got nothing but crickets as a response