You must heal your Pokémon with 65535/4 HP with an item from within battle, which causes its sprite and HP bar will disappear. If you get hit by a damaging move, the HP bar will reappear but the user's sprite won't.
This opens up various move and sprite related glitches, for example, the enemy using Odor Sleuth after getting no sprite and the HP bar showing causes the game to lock up indefinitely.
If you use Charm (or perhaps another unknown move) after getting no sprite and the HP bar showing, then only then you can access some specific sprite glitches.
These work when the user or the foe uses a certain move/does an animation. Many moves cause weird effects. A lot of moves share the same effects, but there seem to be at least 65 or so enemy sprite related glitches accessible with Charm glitch (see documentation link below).
Examples of particular enemy sprite glitches include the foe using Pound making the foe's sprite become small, the foe using Fire Punch making its sprite become small but a little bigger than the Pound effect, the foe's sprite becoming enlarged to fill a square (e.g. Water Gun), the foe's sprite being stretched in (e.g. Encore), the foe's sprite turning upside down (e.g. Metronome) and strange effects with distortions and rotations.
The differences between some of the sprite glitches are hard to spot, e.g. the difference between Dragon Rage and Conversion 2's glitch.
Some moves make the user's sprite temporarily re-appear too. For example, when the foe uses String Shot, the user's sprite will move across the string. Or if the foe uses Absorb, the user's sprite will move towards the foe. Or if the foe uses sprite the user's sprite will appear momentarily where the teeth are, etc.
For a small number of moves, they may be considered as neither foe or user sprite glitches. When Heat Wave is used under the Charm glitch, one of the waves is prevented from moving left for a while but still moves down. When Bullet Seed is used, two seeds appear back near the source they were fired from abnormally and fall.
Some moves 'do nothing' or may restore the foe's sprite back to normal (e.g. Hail).
Spinning block glitch
The spinning block glitch is a sub-glitch of the Charm glitch.
It occurs after Lass Haley (on Route 104)'s Shroomish uses Headbutt with the Charm glitch active.
This causes its sprite to becomes a block that has a spinning effect inside it. The block can spin for a long time until you wait a while, but you can immediately change the sprite back to normal by opening the Pokémon menu or performing another 'missing back-sprite' sprite glitch.