Many PEX crimpers for crimp rings are built like bolt cutters, and you need space to move the handles out to 90 degrees before you can get the leverage for a crimp. You then crimp it evenly and firmly until the tool bottoms out, and then use the gauge to make sure the crimp is good. This means you need to plan ahead to crimp lines located in tight spaces, or buy a compact crimper.
You can also use something like Uponor PEX-A, which requires an expansion tool for the rings, but that tool is much more expensive (maybe as much as $400) than a standard crimp tool.
Bruce