Rush takes place over two nights (7-9PM on both Friday and Saturday) of a predetermined weekend. During that two-hour window on Friday, all IFC houses will be open for rush and PNMs can and should rush multiple houses during the formal process to indicate interest in joining. On the second night, only certain houses that did not fill their rush class on the first night will be open. By visiting a house and signing their roll, a potential new member (PNM) will have "rushed" that house. However, PNMs may only "shake-out" at one house at the end of a formal rush period. This means that a PNM must be at their top choice house by 9 pm. "Shaking-out" is similar to the early decision process in college apps. While not actually binding in this case, it is an indication to the fraternity chosen that they are a PNM's priority house. The advantage of "shaking-out" at your top choice is that it shows commitment to the house being rushed. Houses are looking for dedicated new members to join their tightknit communities, and this indication that a PNM prefers a certain house over others is a strong message. "Shaking-out" is not the only way to receive a bid, but it is by far the most common way of doing so.
Bids can be given out to both individuals that shook out and individuals who expressed interest throughout the term but may not have officially shaken out at that house. It is possible after rushing to receive multiple bids from multiple fraternities. However, a potential new member can only "sink" his bid at one house, indicating his intention to become a member of that fraternity.
After PNMs shake out and "sink" their bid, the current members of the houses will deliberate and then deliver either "bids" or "no bids" to everyone who shook out. The process of delivering bids depends on the house, but you will usually be notified about whether you got a bid a few hours after shaking out. If you get a "yes" bid from a fraternity, they are inviting you to accept a bid at their house and become a member of their organization.