Rashi (Sanhedrin 44b, s.v. D'ba'ya) relates the story of Shimon ben Shetach's capture of 80 witches. He instructed his students to pick up the witches because the sorcery would be powerless against the students if the witches were ungrounded. Is this to say that sorcery does not work unless the practitioner is grounded?
If so, how can we interpret the Midrash (Bamidbar Rabba, 20:20) that states that Bilaam used sorcery to fly through the air?
Perhaps one need be grounded only to cast the spell (as @DoubleAA said here). If so, Bilaam could have cast the spell on the ground. They were then able to fly, which they did.
The case of Rebbi Shimon ben Shetach might have been about their particular method, from where they drew their power.
Also, there are a few types of Kishuf. Rashi points out that there is Latt and Lahatt. Perhaps using demons doesn't require being grounded.
Another possibility is that he was on the ground but projected himself into the air.
Shimon Ben Shetach was the Nasi of the Sanhedrin. Like in Sanhedrin 68a (i think) where it explains that with the death of Rabbi Eliezer witchcraft was forgotten amongst the sages, it is likely that Shimon Ben Shetach (the Nasi in his time) also was familiar with witchcraft. Therefore, perhaps he knew that this particular form of witchcraft required being attached to the ground, where as Bilaam (an "expert" that a King sought out) would probably have spells in his arsenal (?) that wouldn't require being on the ground.
Also, note that Balak himself was an expert in various types of witchcraft/sorcery, but needed to enlist Bilaam due to his specific expertise in a certiain 'field'
I think, in the Rashi on Sanhedrin 44b, the subject of the sentence "He should lift one of them from the ground" is not referring to one of the witches, but to one of the jars that he distributed in the sentence before.
As translated here: http://www.bmv.org.il/shiurim/sanhedrin/san080.html
[Shimon ben Shetach] assembled eighty tall young men and distributed to each of them a jar with a cloak wrapped up inside (it was a rainy day). He also told them to make sure that they were always eighty in number. "When you come inside," he said, "one of you must raise his jar from the ground; from that moment the witches will have no further hold over you; if that does not work then we can never beat them." Shim'on ben-Shataĥ went into the witches' coven and left the young men outside. When the witches asked him who he was he replied that he was a wizard who had come to test them with his wizardry. "What tricks can you do?" they asked. "Despite the fact that it is raining today I can produce eighty young men with dry cloaks!" "Show us!" He went outside and beckoned the young men inside. They removed the cloaks from the jars, put them on, and came into the coven. Thus they bettered the witches, took them outside and strung them all up.
I don't know how lifting one of the jars from the ground would render their powers useless, but maybe that answer lies in what power the witches claimed to have.
