The medium slow experience type is used for all valid Pokémon part of 3-stage evolutionary line excluding the Butterfree, Beedrill, and Dragonite line. Mew is also part of the medium-slow experience type.
The experience calculation for this experience type is as such:
, i.e. 1.2L3 - 15L2 + 100L - 140
For level 0 and level 1 Pokémon the result is a negative number; i.e. -140 for level 0 and -53.8 for level 1 (which is rounded to -54).
However, this is interpreted by the game as an extremely high positive number (16777076 at level 0 and 16777162 at level 1), and the maximum experience is 16777215 (FF FF FF) before the total experience 'wraps around' back to 0.
This means that if a level 0 Pokémon gains less than 140 experience or if a level 1 Pokémon gains less than 54 experience it will jump to level 100 if it is stable. If it gains more than the relevant amount it the experience will be a positive number so the Pokémon will jump to a normal level.
Despite the experience being for an extremely high level, level 100 is the cap. However, note that the level cap may vary for unstable hybrid Pokémon, for example the cap for a Mew/Chansey hybrid with Chansey as the recipient is level 109.
Obtaining level 1 or 0 medium-slow experience type Pokémon
Obtaining level 1 medium-slow growth Pokémon is possible with Special stat encounter Trainer escape glitches, if the enemy Pokémon's attack stage is 1 (i.e. if six Growls have been used).
A medium-slow experience type Pokémon can be obtained at over level 100 using the old man trick and then leveled up to level 0 (by trying to level it up to 'level 256' with Rare Candies).
In Yellow, a Pokémon over level 100 can be hybridized with a medium-slow growth Pokémon that can evolve using the Pokémon merge glitch, and then the Pokémon can be evolved with a Rare Candy.
Any Pokémon with a negative experience value that is stored in the PC will immediately become "stuck," attempting to withdraw this Pokémon will cause a lockup. Here is a discussion from the forums https://forums.glitchcity.info/index.php?topic=6588.0.html on glitch Pokémon experience types. Check under RB medium-slow families and Y medium-slow families as they will cause this lockup.
Similar to the withdrawal lockup, any Pokémon with a negative experience value given to the Daycare man, and taken out before the experience becomes positive, will cause a lockup.