Christmann himself wrestled with the banjo, also a popular New Orleans instrument, before settling on the bone challenge. He became curious about what was being called free jazz after hearing some recordings by John Coltrane and Ornette Coleman. In the late '60s he began performing in European groups that were creating their own variations on this noisy, freewheeling music that was sometimes accused of being angry or full of hate. The instrumental effects the trombonist would have to master in order to carry on along with everyone else in this kind of music were strikingly similar to the type of overblowing and slurry slides the New Orleans trombonists were fond of, as were the soloists in later big bands such as those led by Duke Ellington or Fletcher Henderson. Like these early trombonists, Christmann utilized an arsenal of mutes and plungers in order to get different tones of out his horn. He played with Frankfurt tenor saxophonist, clarinetist, and accordion player Rudiger Carl. Christmann also worked in several quintets under the leadership of bassist Peter Kowald between 1972 and 1974, tiring times indeed as this bandleader was famous for driving immediately home after the last gig of a tour, no matter how far away one happens to be and no matter how late it is. This bleary-eyed act of desperation is known as "pulling a Kowaldski." These groups were also some of the early improvising experiences of Aachen drummer Paul Lovens, who has played expertly off and on over the years with Christmann in various settings from duo on up.

From 1972 to 1981, Christmann established a regular duet with percussionist Detlef Schonenberg. The duo worked in collaboration with dancers but also made several recordings that were remarkably influential among other improvisers, such as We Play on FMP. The trombone seemed in some ways an ideally expressive instrument in a music that derived its logic from partially related bleeps and splats. He made his first collaborations with electronic music through synthesizer player Harald Boje. He joined the large Globe Unity Orchestra, a project under the loose leadership of pianist Alexander Schlippenbach that has involved many European and American improvisers over the years, including the young (not the old) George Lewis. This large group also recorded some compositions Christmann wrote for them, but not with any grand degree of accuracy. In 1975 he began performing as an unaccompanied soloist in an era when many other horn players were doing similar things, infuriating travel agents used to booking whole bands as well as audiences frightened by the whole idea. In a difficult idiom, Christmann was one of the great players, dramatically using space and silence but unleashing enough blustery trombonics to drown an elephant if need be. His use of the plunger and assorted mutes was superb and many other trombonists copied his techniques. From 1978 through 1981 he performed and recorded in a duo with the American expatriate cellist Tristan Honsinger, also resulting in some of his best work as this was a well-conceived combination of players, the cellist's florid and almost hysterical manner taking Christmann right over the edge. From the late '70s onward he followed the trend toward loose conglomerations of players that could fill up a concert evening with improvisations in various combinations. His version was Vario, which was presented at a variety of European festival events and recorded for Moers Music. This group included Lovens as well as British vocalist Maggie Nichols and guitarist John Russell.

In 1982, Christmann developed a presentation entitled Deja Vu in which he performed in combination with film production, electronic music, lighting effects, and contributions from bassist Torsten Muller. It was not received that warmly, with critics encouraging Christmann to return to the simpler times of just his trombone. Since the late '90s he has been concentrating much more on special projects to be realized out of his home base in Hannover and has severely limited his touring activities.