how

COB can happen during year when it is not formal recruitment.. If they have an opening in their house (for whatever reason, graduating, someone drops, someone transfers, whatever), they might be in a position to offer a bid. They can then have someone they like over to meet more sisters with that in mind. Often you have to know people in that house and/or be someone that they are already interested in as a sister. For example, if you know there is a spot, you could invite over your favorite lab partner to meet sisters and see if she might be a good fit.

Spring recruitment takes place for similar reasons as COB. If a house has some spots open, they may participate in a mini version of formal recruitment in the Spring. You never know who might participate until closer in time, but it is not everyone and it can end up being a hodge podge mix of houses. Like COB, it helps to know girls in the houses you are interested in, since there usually is not a too many spots and some houses may already have certain people in mind.