Maybe he is just saying its a bad idea to leave your weapon out/embedded. (which EA already pointed out)
Presumably you might need to be stabbing other people and leaving your weapon in someone's body for any length of time might not be a good idea. (Which EA already pointed out)
Might, MIGHT, also have something to do with armor. Might be considerably harder to withdraw a spear from deep in a human body AND the armor around it. The "in" thrust might be easier for several reasons. Don't know enough about Japanese armor but some forms of "western" armor tend to "grip" the blades/etc. Not hard at all to get the weapon "hung up"--depending on the weapon and the armor.
In terms of karate---maybe its similar---can't figure out why the retraction would add power to the strike. But getting the striking limb back asap would:
A-Allow you to strike again as fast as possible.
B-Less chance to be grabbed/grappled.
C-"Posing" your strikes is a really bad idea
Chris Thomas
"While people are entitled to their illusions, they are not entitled to a limitless enjoyment of them and they are not entitled to impose them upon others."
"Team Cynicism" MVP 2005-2006
Currently on "Injured/Reserve" list due to a scathing Sarcasm pile-up.