This is a defensive movement used to avoid a jab or cross coming from your opponent
Combination using the slip
A good habit to develop is using a technical drill for sparring. In this case, to practice the slip, try a few rounds where you and your opponent can only throw jabs and after every jab you must slip or duck, this is a great way to ingrain this into your movement to make it automatic. As the most oft-used punch, it stands to reason the jab will also be the most oft-countered punch so learning to move your head after throwing the jab is essential to your success.
Boxing Coach Strength and Conditioning Coach Boxing Writer for the Ringside Report Boxing Author of: (Available for download on Amazon) The Boxing Cheat Sheet - Your Ultimate Guide to Ring Survival Strength and Conditioning for Boxing - Work out Hits to get you Fighting Fit! Forgotten Legends of the Ring - Ten Past Masters of the Squared Circle