Lifters will often allow their bodies to sag on dips with little to no core activation. Instead, focus on getting tall and having your torso in front and over your hands rather than in back of them. Get as "tall" as possible over the dip handles rather than sitting below them. One of the best dip variations to nail this cue is straight bar dips.
If there's any sagging, lack of hip hinge or failure to hollow the core, the lifter's stomach will run straight into the bar, minimizing movement. If performed perfectly, the movement pattern will be identical to standard dips, including the same optimal range of motion. Just be prepared to fire your core to the max.