This was also shown to be how he transformed back in the 2002 series, and in the comics.
You can see him using it in the 2002 series here:
Go to 10:40 to see the transformation.
It should also be noted that instances of enough raw energy or magical force being thrown at him directly have on rare occasion seemed to "knock" him out of his transformed state. This too can be seen in the 2002 series.
Go to 16:00 - 16:55 to see the full effect. Even the sorceress knows it can happen, as she yells out "He-Man, no! You must not challenge the power of the Stone!" So this would indicate there are some unforeseen magical conditions which can change him back without him willing it.
Another example of this would be in the 1980s cartoon, when he travels to Orko's dimension, Trolla, as He-Man, but is reverted back to Prince Adam as he steps out of the gate. He was unable to change back, even with the sword. After seeing Orko's magic work flawlessly and how the normal laws of physics seemed in opposition in that realm, it was later deduced by Cringer that "everything is backwards here!", so Adam had to recite his incantation backwards in order to utilize his full powers.... though why that didn't make him the weakest man in the Universe is beyond me.