If the spindle is on when you hit the high/low range switch the spindle will come to a stop. It will change to high or low range then the spindle will start again on its own.
I really don't know what changes when it goes from high range to low range. In low range you really do not have a lot of torque at slow speeds. It is easy to stop the spindle when drilling with a 1 1/2 inch drill at anything below 100 rpm.
The biggest thing is to be careful not to run the machine beyond the speed rating of the chuck you are using.
If your machine doesn't run at all in low range I have no idea how to fix that.