His defense rests without mounting any defense at all.
He is either:
1. Falling on his sword for his Dear Leader,
2. Protecting his family from drinking Russian Polonium tea
3. or has been paid off. (Family gets to live well, while he spends the rest of his life in prison.)
The funny thing is that if it is #1, and Trump pardons him, that opens up Trump to some seriously vicious investigation. It'll make the Mueller investigation look like a Girl Scout cookout.
Just because he doesn't put up a defense doesn't mean his defense team is giving up. It simply means that they think that they will suffer more damage on cross than they will gain advantage on direct.
The prosecution must prove guilt. The defense doesn't need to prove innocence. If their best strategy is to prove reasonable doubt through cross-examining prosecution witnesses only, they won't present a defense at all.
If Manafort were my client I wouldn't want him on the stand. Would you?