Furious George explains it better than I ever could. This is from the sticky he created:
"Deca (a 19Nor derivative of test) is a very anabolic drug with little androgenic/estrogenic qualities and has a side effect of aiding the joints and immune function. This adds mass and strength without the androgenic/estrogenic sides. This makes Deca mild in terms of hairloss, Benign Prostate Hypertrophy (BHP), blood pressure, lipid levels, and gyno (although 19-Nors are able to elevate prolactin levels which may lead to gyno as well). Deca converts to Nor-Estrogen which acts much more weakly on the receptors so gyno is less of an issue unless very high doses are used. Nor-Estrogen is believed by many to be the reason for Deca’s ability to aid joint lubrication/health while on.
Deca has also been shown to aid in collegen synthesis and bone density. Deca is commonly used by those who are suffering from previous injury or illness so it makes sense to use it in a long cycle where their will be continuous stress on the joints, bones, and ligaments.
The Decanoate ester produces stable levels even when only shot once a week and because of it’s side effects of improving joint health and immune function it is a great addition to a long cycle where the body will be under prolonged stress.
Deca does have a very dramatic negative effect on libido due to it’s 19-Nor structure (able to activate the progesterone receptor) so dose is typically lower than test at a rate of 1/2 to 2/3 that of the Test and use often restricted to just a portion of the cycle (ending at least 2-3 weeks prior to Test). Deca is often combined with Winstrol to offset the progesterone receptor effects it has or to counter the prolactin effects it is combined with cabergoline or bromocriptine."
Ask Saps, as I’m also taking this from him: Deca dick is no myth.
The higher levels of test allow you to keep your libido functioning while taking the 19-Nor. Since the deca will shut down your HPTA, the use of the stasis/taper gives your body a much better chance of a smooth transition to homeostasis.