What does shift+open do?

holding down SHIFT while opening will reset the defaults of many Apple built in software packages in OS X/Mac OS. It works because some software packages may have a corrupt preference or setup file and resetting the file can repair the problem.

Some 3rd party apps such as Adobe CS requires SHIFT+OPT+CMD+CTRL to reset the applications, and some packages do not have any such feature to accommodate this.

Shift+click on any UI element to run the animation in slow motion (shift click green, orange, red window icons for example).

I suspect you are thinking about ctrl+clicking on a Application & then selecting the 'open' option.

That is part of Gatekeeper…

OS X: About Gatekeeper - Apple Support

See the section…

How to open an app from a unidentified developer and exempt it from Gatekeeper

Basically the OS does not allow you to open an application that does not have a signed certificate from Apple. You can add an exemption by ctrl+clicking to 'open' the app. You only need to do that once (unless the app gets updated).

Generally it is safer to avoid adding too many exemptions but you can't avoid it if you use a lot of apps from outside the Mac App store.

P.S. 'ctrl+click' is the same as a right click if you have that enabled.

Shift-click will stop the app from restoring the last session. In some apps it will also clear preferences and is akin to starting in Safe Mode.

If you are force quitting often you should ask why. Post a new topic on the forums with an Etrecheck report if you want to spend time investigating any issues that may be visible to the users here…

http://etrecheck.com/

Disk failure or incompatible apps may be causing problems that could be fixed or addressed before data is lost or damaged etc.

Don't do a force quit unless you have no other choice.