contracting abs...

[enigma10]

When do u guys contract your abs during working out abs?? I've been told when exhaleing to squeeze my stomach as hard as i can, but do i keep my stomach tight the whole time ?

[eatrainrest]

i try to do that with most muscles

[FireGuy]

Exhale, curl the spine.

[enigma10]

lets say I'm doing crunches, do i A stick in my stomach in as best as i can the whole time or B only stick my stomach in while exhaling ?

thanks