The Substitute drain move not missing is a minor battle glitch in English Pokémon Red, Blue, and Yellow, where HP draining moves don't miss against a Subsitute, even though they should (and they do in Pokémon Red, Green, and Pokémon Blue (Japanese) as well as in Pokémon Yellow (Japanese)).
How to perform
When a Pokémon is under the effect of the attack Substitute and the opposing Pokémon uses an HP-draining move (such as Absorb, Mega Drain or Dream Eater), the move may hit.
Note that this glitch does not exist in any of the Japanese games.
Source : Pokéred, Pokéyellow
[de] contains the move effect, which makes the SWIFT_EFFECT check successful.
However, CheckTargetSubstitute overwrites a with either 0 or 1, so the following comparisons fail.