Q: How to precisely set trim points in quicktime player el capitan

Is there a hidden secret way to set the end and beginning frames of Trim in QuickTime Player 10.4?

Not really. The QT X Player is simply not a frame/field accurate trim editor. While short clips and player windows having longer "Trim" bars are more easily trimmed with greater accuracy, use a QT 7 based app (like QT 7 Pro or MPEG Streamclip) or a video editing app for trimming with frame/field level accuracy.