Ignition system guidance
 

Hey how's it going, havent posted on this site in years, but im getting back into rotaries and just wanted to continue where i left off, i have a few questions

the repair manual for the 13b-rew engine says

1.trailing 20 atdc(-20 btdc)
2.leading 5 atdc (-5 btdc)
3. how does the ignition cycle work on a stock 13b-rew engine between the 2 rotors?
4. when the first rotor is at tdc is the rear rotor 120 degrees apart from it?

The way i see it (considering we are starting right at tdc of the front rotor)
1. leadings fire -5 degrees btdc on both rotors at the same time.
2. Trailings fire 20 degrees atdc on the first rotor (why does it say -20btdc in brackets)
3. then the leadings fire again 5 degrees atdc in the middle of the power stroke
4. i know 3x the e shaft spins the rotor spins once. can someone please explain the degrees for both rotors and all the cycles. i have a hard time wrapping my head around this.

if someone wants to break down the whole cycle between the 2 rotors that would be awesome, i just looked at diagrams for 1 rotor at a time. if i am totally off please help me get this down, i really need to understand this from the intake stroke to the exhaust stroke with all the degrees listed for the eccentric shaft.