gsap.registerEase() enables you to add or register custom eases with GSAP. These tweens can then be used as any other ease in tweens and timelines. These eases generally return values between 0 and 1 but can go outside of that range (like GSAP’s elastic ease).

For example:

gsap.registerEase({
    name: "myCustomEase",
    ease: function(t, e) {
        //ease timing here
    }
});