Motor that can wind up and do a sudden "release".

I am looking for a system that could pull something in and then suddenly release it (like a slingshot).
Normally I would use a system where the gear has one side that is cut off, but I need different "power levels", meaning:

The idea is that you hold a key for a certain amount of time, the longer you hold it, the tighter the slingshot gets. When you release it gets released immediately.

Anyone have a clue?

My suggestion would be to have separate mechanisms for 'pull' and 'release'.

For 'pull' you can use a screw thread to 'pull' a nut such as often used in garage door openers.or a continuous belt or chain such as used in a roller-coaster.

For 'release' the moving part can have a latch that engages the springy part. The release would disengage the latch. I solenoid might work for that.

Is this specifically a slingshot (probably end up looking more like a crossbow) or are you making a general purpose object launching device? I can understand your desire to not reveal all of the details but it makes it really difficult to give an appropriate answer.

No worries mate, I'm glad that someone is willing to think with me on this!

The details:

We're building a project where a computer could choose a number as strength. This number will determine how hard the spring will be pushed in. Upon reaching the chosen strength (or pressing a button, doesn't really matter) the mechanism should fly forward (like a slingshot) to hit something which in return will start its course (in this case, a small rollercoaster vehicle).

I already thought about creating an automatic linear slide with a mechanism (servo, solenoid, ...) that can move itsself out of the way to release a rubber but I was wondering if there was a motor that can release itsself.

The object that needs to be pushed isn't that heavy: think about a small toy car (matchbox-alike).