You make/join a game. One that's low on people. Pick regulus. After others join, sit there for second.
A) Some jackass on your team will pick regulus also, then instantly hit "ready". He will then ignore "no doubles" or questions/comments/concerns/hate directed at him for the next 2 mins. I envision some guy sitting behind his computer, eyes squeezed shut, praying "letmeberegulus, letmeberegulus, letmeberegulus".
or they don't pick regulus. You're blocking him, and he is writing your name on his "list" of people to kill. Now that the game filled up, you choose UB or something useful. Then we go to:
He instantly picks regulus, once you cleared the "block", and then hits ready instantly. He is now on the moral highground of picking regulus. Any attempts to change his mind will result in the praying thing again, or him typing "why not?" and stalling his ass off, while SECRETLY praying "letmeberegulus".
If your teammate fails either option of the test, I would suggest getting out of the game (or kicking if the host). Let's face it. 99% of regulii players are passive aggressive weiners that want to avoid engaging the enemy at all costs. It's stressful. They have to buy HP items, micro, use potions, know what the items do, etc. It's easier to let the "big boys" deal with it, expecting them to deal with 2v3.
Reminds me of Company of Heroes. In 2v2 or higher, you'd get that teammate that would not engage the enemy, saving up for artillery. They would expect you to fend off a 2v1, then he would ride in a save the day with artillery. What you call a "turtle".