Under starter’s orders
Definition 1 of 2
Pro Tip 1/3Origin in Racing
Originally used in horse racing, referring to racers preparing at the start line. 
At the racetrack, horses were under starter’s orders and the tension was palpable.
Pro Tip 2/3Imminent Action
Indicates immediate readiness, suggesting action will happen any second. 
The swim team stood under starter's orders, ready for the signal.
Pro Tip 3/3Rigorous Preparation
Implies thorough preparation before the start of a race or competition. 
After months of training, the athletes were under starter's orders, awaiting the whistle.