On Monday, November 5th in Los Angeles from 1-4pm (presentations start at 2pm) the latest batch of HMC INQ-funded founders present their companies to a room full of specially selected investors and press. Presenations are three minutes each.
Then, on Thursday, November 8th (same times) in San Francisco we'll have another Demo Day!
(There's a 5C-only Demo Day Sunday, November 4th at Mudd from 3-5pm.)
When you decide to invest in a company you meet at Demo Day, they will follow the same Handshake Protocol that Y Combinator uses.
In a nutshell, this just means that once you shake hands (or fist bump) on a deal ("$X via YC SAFE at a $Y valuation cap), the company will send you a quick email confirming it.
Just reply yes to that email, and the deal is sealed. (Actually signing the SAFE and sending the funds can then happen over the next few weeks.)