Slip It In is an extension of the sound Black Flag utilized on its predecessor My War: heavy, cathartic, intense, dense and progressive. At this point, Black Flag was considered by many to be one of the leading bands of the American punk scene. The album pursued the newer, lengthier song arrangements that Black Flag would develop until its demise. It also features Henry Rollins' further development as a songwriter, contributing four of eight tracks on the album. This album also demonstrates Black Flag’s increasing use of instrumentals, where Greg Ginn demonstrates his increasingly more complex playing style.
Slip It In was recorded on a brief break on the continuous tour for My War, My War saw Black Flag at their most ambitious. This year they would release three full-length albums, and toured nearly constantly, with Rollins noting 178 performances for the year, and about that many for 1985. With Dukowski gone, Ginn ceded much of the spotlight to Rollins, who had expressed some discomfort over being the group's de facto spokesman, while Ginn was the recognized leader (Ginn wrote the majority of the group's songs and lyrics).
In many ways, Slip It In is a fusion of the styles utilized on their previous releases (with vocals), Damaged and My War. The songs are inspired by heavy metal, yet the material also shows traces of free jazz, punk rock and, particularly on the track "My Ghetto", even contains traces of a musical style that would later become known as Powerviolence. At the middle point of the album, there is an instrumental track called "Obliteration" which highlights Ginn's chord progressions where Brandon Sideleau of Punknews.org claims that it "...mashes sludge and jazz into an ominous hybrid." One critic writes that Slip It In "blurs the line between moronic punk and moronic metal". However, despite various criticisms, the album is very highly regarded in the underground music community and is often considered one of their best works.
A screenshot of the "Slip It In" music video, one of the few music videos by Black Flag
A low budget music video was produced for the album's title track. The music video revolves around a teacher lip syncing to the vocals to the song along with the class and cuts to clips to the band performing to the song. The video ends with the teacher telling about the album coming out and promoting their "students" to see them on their tour.