Based on experience, it is safer with respect to the LP, to turn the tt off while mounting the LP and clamping it. If you slip slightly the spinning platter can push the LP into the tone arm or out of your hands onto the stand or floor. Happens rarely, but turning the tt reduces the consequences of the slip.
Is your turntable spinning while you clamp down?
I just got back into vinyl and I bought an inexpensive KAB clamp. So far, so good, but putting and removing the clamp can be a pain. What is the proper method? Do I turn off the turntable, change the clamp, or is it ok to clamp down and partially stop the turntable from spinning while I do this? My turntable is belt driven and I am wondering what clamping while the turntable is spinning will do to the motor and/or the belt. Is it my clamp? Can someone help an analog newbie?