So I've returned to this game I guess! I haven't really seen any discussion of techs or strats or... anything really, so I figured I'd share some of my (general, mostly non-level-specific) findings here. Most of this will probably work on all versions, but I'm basing it mostly off of the PAL PS2 version. Once I give the GC and NTSC versions more than a brief look, I'll make a thread on version differences as well.

It's also my first SRC forum post, so here's hoping I don't mess the formatting up:

[section=Jump Cancel] Named after the similar tech from the later Castlevania games. The basic idea is that landing from a jump and rapidly pressing Fire at the right time lets you fire multiple shots in quick succession - however, this is pretty difficult to pull off and mostly boils down to random jumping, button mashing and praying.

With the right timing, this can bypass the reload animation, which happens after quick 6-shot bursts for stock ammo (or 3 orange "whirl" shots, or 1 rocket)

[section=Propulsor Spam] Or Jet-spam as I like to call it. You just mash Jump while in mid-air. Your horizontal movement is slower than simply holding it down (I think?) but you cover more distance because you fall slower.

Speaking of movement speed, my (not very thorough) research has discovered that the fastest form of movement is simply running straight! Followed by jumping and then lock-on. Not entirely sure where the jet stands in terms of horizontal speed.

[section=Quick Ledge Grab] PK can't grab ledges on his way up in a jump. Rather than waiting for him to start falling, you can save a few frames by tapping Jump at the correct time, since the propulsor cancels your vertical momentum. This is pretty finicky however, and mistiming can either make you grab the ledge slower because of the vertical boost, or miss the ledge entirely.

While on the subject of ledges, jumping is of course faster than pressing Up to climb up, but another correctly timed jet can land you on the ground faster than normal in case you want to immediately jump again or start running.

[section=Tanking Shock damage] Normally PK has no animation for taking hits, but electric damage will stun him for a second. You won't be stunned when you're swinging from Swingports however, so you can tank the hits without losing time. Most noticeable on the Mothership level on the corridor before Checkpoint #3.

[section=Cutscene Jump] Skippable in-game cutscenes remember your position where you triggered them, and afterwards briefly place you back there even though you fell down at the beginning of the cutscene (So I guess the enemy introduction cutscenes are the only ones that apply?)

For a brief moment (a frame or two?) PK is in a standing state, meaning you can jump off of it, even if you're in midair. The only place I found a use for this is Area 52's Coolflame 2 (the red one's) introduction scene, where if you activate the cutscene at the highest point at the left, and pull off the jump (combined with jet-spam), you can reach the ledge to your left, saving... well, only about 10 seconds or so, and skipping Scientist #1.

I couldn't get this on my run, so here's a clip of it in action:

(Here's it being useless in Level 2: youtube.com/watch?v=XgeeIm5Os8c, but at least I found out that if you trigger the cutscene to the left, the Evronian's AI won't trigger or something.)