Tape motion on many systems is bidirectional, i.e. tape can be read either forward or backward at the request of the controlling computer. Because the supply vacuum column keeps a small, constant tension in the reverse direction, the capstan can feed backwards without the tape bunching up or jumping out of its path. Unlike most audio tape systems, the capstan and head assemblies are always in contact with the tape, even during fast forward and reverse operations, only moving the head assembly away from the tape path during high-speed rewind. On some units, manufacturers provided a "fast search" capability which can move the tape quickly a certain number of blocks, then bring the tape to a halt and go back to read the requested data at normal speed.