Hand over hand you will cross your arms over and usually -only- pull. Push pull both hands will stay on their respective sides of the wheel.
Push pull is much safer, if you get into an accident. Reason being is that if your arm is over the wheel when the airbag explodes, there is a high chance of you whacking yourself in the head. Airbags are violent things, so it's quite possible to severely injure yourself.
At a defensive driving course I was taught something slightly different again. Both hands stay on their respective sides of the wheel, but you only ever pull. Say you want to turn right, you would move your right hand up and pull down. To exit out of the turn reach up with your left hand and pull down. Never let the wheel spin back by itself, and only ever be turning the wheel with one hand.