XRAY - Model racing cars

what does changing your shock rebound actually do? I usually run about 50%. My normal track i run on is rough, bumpy, dusty and medium to low traction. The are also 2 big jumps and some woops. Am i running the right rebound? Here is some of the rest of my set-up too.let me know what you thinks, and if anyone has any suggestions.my buggy does o.k. but it seems loose, and needs a little more turn in off power plus it doesnt seem to handle the 2 woops and bump sections to good.

5-5-3 diffs
30wt/1.3 pistons/ silver springs
2 deg. front camber
2deg rear camber
4 deg rear toe
1 deg front toe out
12 deg caster
3 deg anti squat
stock kick up
stock sawy bars.
medium wheel base
30mm rear, 29mm front ride height

Rocco isn't 103 front and 113/144 mm shock length too short for bumpy tracks ?! Because it has almost no downtravel for the suspension to work with holes,bumps,etc.