Warm approach
A warm approach is a PUA term that described romantically approaching someone that one is already acquainted with instead of approaching a stranger.
Warm approaches seem to be the most common way people initiate romantic relationships (in societies with free-mate choice). Research indicates that most people (of both sexes) prefer to make romantic overtures to people they already know.[1]
As incels are often socially isolated, the inability to make warm approaches, where their chances of success may be higher, may often be a large contributing factor to their inceldom. Supporting this, some studies have suggested that there may be a general tendency for women to wait longer to transition to sex when their partners are less physically attractive (and this may extend to other desirable traits).[2]
These threads of evidence suggest that most men need to generally invest heavily in a woman (in terms of resources, time and entertainment value, in order to outwait her natural coyness. They need to build trust, generally present themselves as a suitable mate, outcompete other men for her affections, and take an indirect approach before she will accept a relationship with him and provide him with sex. This essentially means that lower-value men are often limited to warm-approaches, while men with high mate value may be able to effectively pursue short-term mating strategies more easily, as women will likely be more receptive to more aggressive and overt sexual advances on their behalf.