Flanking is an odd term, but I think maybe I can explain how they got to it. . . you understand the enemy's rear and sides are his flanks - thus if you make a flanking attack, you're mobile enough to whack him from behind in standard combat. (subs do some nifty stuff along these lines in TRW) You'd also be mobile enough that if your enemy bit back, you could dart away faster, thus the retreat ability.
As for pinch, simple enough. Historically, gunpowder and any balistic weapons are ineffective at very close range: the firing line is under attack and can't put out waves of projectiles. Thus, in some cases where ducking won't work, the opposing non-projectile forces would try to very rapidly close or pinch the gap, thus breaking up any wave of missiles and maybe greasing a couple of crossbowmen. Not ever a first-line idea, but if you're on a field, you're close, and ain't nothing to hide behind, try to lop off his head. . .