hay is picked up by a the pickup tines, then pushed perpendicularly by a "fork". it is then compacted into slabs by the plunger and tied with twine by the knotters. the bale then comes out the back of the baler.
Knotters to tie twine were developed at the latter part of the 19th century to tie cereal sheaves produced by binders. In principle the same system is still in use today in balers used for hay,...