help with Avisynth Terminology

[bazz]

can someone explain or link me to some guide to help me understand overshoot and undershoot.
Ive searched gooogle , read the Wiki pages and i can not find any answers.

ive been doing alot of small X264 rips from 720p sources and XviD rips from 720p or DVDR

so ive been trying out different scripts i find on google to denoise and sharpen, running off small samples to decide which is better.

Ive had some success with this one i found on a thread in this forum

FFT3dGPU(sigma=1.5, bt=3, bw=32, bh=32, ow=16, oh=16)
MT("""LSFmod(smode=5)""", threads=4, overlap=8)

this is fine but its about time i understood the scripts better and made my own scripts, in this script above ive been using
ive found all the info on sigma , mode etc but i dont understand
overlap ?
and threads, how many threads can be used?

any help , any pointers would be appreciated..thanks