Changes the encoding of self to encoding, which may be a string encoding name or an Encoding object; returns self:

s = 'łał'
s.bytes                   # => [197, 130, 97, 197, 130]
s.encoding                # => #<Encoding:UTF-8>
s.force_encoding('ascii') # => "\xC5\x82a\xC5\x82"
s.encoding                # => #<Encoding:US-ASCII>

Does not change the underlying bytes:

s.bytes                   # => [197, 130, 97, 197, 130]

Makes the change even if the given encoding is invalid for self (as is the change above):

s.valid_encoding?                 # => false
s.force_encoding(Encoding::UTF_8) # => "łał"
s.valid_encoding?                 # => true