The typical pop-up engine gliders come in two varieties - sustainers, these are smaller less powerful engines which will typically give some height gain and get you back home, and self-launchers, these are more powerful and, as the name suggests, can launch the gliders.
The former are usually air started by doing a gentle dive, pull the decompression lever, allowing the prop/engine to spin up and when you then let go the engine will run (usually)
The self-launchers typically have an electric start.
Either system is not always as reliable as one would wish, and people do get caught out by relying on the functioning of the engine. Our MO was to go for the engine sooner rather than later and make sure you were in an area where landing out would not end up in damage of the glider or its occupants.
Having an engine revolutionises the gliding cross country experience - being able to plan something at the end of a flying day is nicer than having to wait (for hours) for one's retrieval crew. That is very much more a young person's game where time is not an issue.