When the mold opens the horn/angle pins allow the slide to motion away from the cavity and the slide retainer aids in locking the slide in place to allow the ejection of the plastic part. The purpose of a slide retainer serves 2 purposes.
If the slide moves forward during the ejection phase of the cycle, the angle/horn pin will crash into the top of slide and could cause mold or press damage. The proper size of the slide retainer is determined by the weight of the slide being held, in some cases a slide will require two retainers to hold the slide in position especially when the slide is on the top of the mold. During the design phase the mold designer will do a feasibility study by rotating the part around in the mold base to place the slides either on the side or bottom of the part/mold to avoid the effects of gravity and to protect the slide from coming forward as a secondary safeguard. When the slide is in the close position during the injection phase of the molding cycle, the injection press will apply a large amount of pressure on the slide that will push the slide backwards. In order to hold the slide in the forward/molding position, an item called heel blocks are there to assist. The heel block are on the opposite half of the mold, when the mold is closed the heel blocks wedges the slide forward so it can withstand this pressure without movement.
Slides are typically used to produce features in the plastic part being molded for example: holes, recesses, clips, or undercuts.