Green’s Dictionary of Slang

1. to reverse one’s plea in court.

1970		Canberra Times (ACT) 28 Oct. 40/3: I asked him how he thought he would go. ‘He said he had a good chance of beating King Pedro, but thought he would somersault over Big Philou’ .
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 45: Somersault Change one’s plea in court.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Somersault. 1. To change a plea in court.

2. to happily take both the passive and active roles in homosexual relationships.

1984		B. Ellem Doing Time 129: And then you get the guys who’ll somersault: they’ll be either a cat or a hock depending on the situation.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Somersault. 2. To adapt to either role in a homosexual relationship.