It's not always spirits.

Sometimes is inner alchemy, and the 'magical effect' is produced through etheric or astral resonance.

in that Fantasia movie

When the apprentice (perception) ignores the source (perceivable), then magic (suggested) gets a life on its own.

to prove spiritual

Proving implies "making credible" aka tempting ones creed to hold onto the suggested proof, while ignoring that spiritual implies "animated".

Being alive implies being (life) animated (inception towards death), hence proof to be found only in the pudding/putting.

harmless

Only during harm (inception towards death) can there be peace/piece (life)...reasoning (more vs less) tempts one to ignore that.

a pact with a negative entity, you can't see, that moves the matter

a) Consenting to a suggestion implies entering a pact among entities, which in return prevents the consenting entity to see perceivable.

b) Ignoring perceivable by holding onto suggested prevents ones self discernment of being matter (life) within motion (inception towards death).

c) Those who consent (agree or disagree) to a suggestion put themselves into a conflict of reason (agree vs disagree), which those who suggest are permitted to rebrand into for example...positive vs negative.

how magicians do that

Suggestion tempts ignorance of perception. Consenting to a suggestion makes the magic happen, hence establishing a fictitious happen-stance within ignored reality, which is being shaped by those who suggest it.

One cannot be a magician until another consents to ones suggestion.