On factory M10 cylinder heads, the rockers are held in position by a spring. At high RPM’s, that spring may allow the rocker to move in relation to the valve stem (sometimes enough to walk off the valve stem) which quickly results in a broken rocker arm. With rocker arm locks, you eliminate the spring and positively locate the rocker arm. Locks also make head assembly simpler than with thrust springs. We use these on all of our performance engines and cylinder heads.