Remember the 100m race at school Sports Day ?
Emerging at a T-junction, or onto a roundabout, starts like that. (After all, a roundabout is just an efficient type of T-junction onto a curving one-way road).
At the race, when the starting pistol's raised (= a usable gap in traffic), raise your body and tense your legs (= set gas, get the biting point, and have your hand ready to release the handbrake).
When the gun fires (= the gap arrives), LAUNCH yourself out, THEN EASE OFF so you can steer / unsteer.