The mechanism is to watch the banks you have money in. A company still has to pay it's bills. To pay bills, you need some money in a bank, it's unavoidable.
So, let's say you are a company with 4 banks accounts. Each has $500k in it. One of them is SVB. You probably just move the $500k into one of the other bank accounts. It's no big deal per se, but you do it. That's a run on the bank if lots of companies do the same thing.