Should pressure only be applied at the top of a pastry bag?
Usually you would twist the top of the bag (hint: don’t overfill) to close the bag and to get a “basic pressure level”, which will depend on the consistency of whatever is in the bag. Somewhat taut, but not leaking out of the nozzle.

Then you hold the bag closed at the twist part with thumb and forefinger, a bit like a ok-sign, just closer. Gradually closing the fingers, more sequentially that simultaneously, allows you to be very precise with the amount of pressure and thus with the flow at the nozzle. Twist the top further every once in a while to keep the bag tight. 

If you are clenching the bag somewhere in the middle, you will 

a) lose the “basic pressure level” after some of the batter/cream/icing was squeezed out
b) have a harder time controlling the “flow (which will for example influence the “tips” of a swirl or the top of a macaroon),
c) risk the top opening up and the filling overflowing and
d) warm up the content of the bag, which is often undesirable, changing the texture and at worst, melting it.

There’s a very good and practical reason to practice using only one hand - you need the other to maneuver the cupcakes/cookies/cake you are decorating. While for cupcakes you may get a decent enough swirl with a “two-handed clutch” method, there are more advanced skills like piping flowers on a flower nail where you really need the extra degrees of freedom.