Question

How do factors such as lowering HP, paralysis, etc change catch rate?

Answer 1

The system uses a formula like this to operate catch rate:

• HP max is the number of hit points the Pokémon has at full health
• HP current is the number of hit points the Pokémon has at the moment
• Rate is the catch rate of the Pokémon (which may have been previously modified from the use of the Heavy Ball or Safari Zone modifiers)
• Bonus Ball is the multiplier for the Poké Ball used, and
• Bonus Status is the multiplier for any status ailment the Pokémon has (2 for sleep and freeze, 1.5 for paralyze, poison and burn, and 1 otherwise).

All from Bulbapedia

Answer 2

Well Status Problems Help Due To The fact That Most of them immobilize the Pokemon your trying to catch
like sleep: you cant struggle while sleeping
this applies pretty much the same with other status problems
the HP lowering system works like: If you run you get tired and cant run anymore
same with pokemon when you lower a pokemon's health they get tired and will not struggle that is why the "Faint' because they are so tired

so when trying to catch pokemon Al you need to do is cause A Status problem ( I Would use sleep)
then if playing pokemon white use false swipe on weaker pokemon because it leaves it with 1HP
or just cause continual damage until red or yellow is the color of the opposing poke's health then use a ULTRA BALL.