What is the function of the Hyoepiglottic ligament?
It's an interesting question. This 2010 paper contains some discussion of the function of the hyoepiglottic ligament. The following is a quote from the introduction:


  The epiglottis is connected to the
  hyoid bone by the hyoepiglottic ligament (ligamentum
  hyoepiglotticum), which is a connective tissue structure that
  serves as the roof of the paraglottic space and the
  preepiglottic space. Anatomically, the hyoepiglottic ligament
  separates the supraglottic larynx from the base of the
  tongue. Some clinical studies have shown that this
  ligament inhibits the spread and invasion of carcinoma... The hyoepiglottic ligament is elongated by
  approximation of the thyroid to the hyoid during swallowing.
  Tension on the ligament results in a “snap” retroversion
  of the epiglottis. Thus, the folded epiglottis represents a
  state of higher energy than that represented by the unfolded
  epiglottis; relaxation of the tension on the hyoepiglottic
  ligament releases the cartilage to automatically return to its
  normal resting position [emphasis mine].


In the study, the authors demonstrate histologically that the hyoepiglottic ligament is bipartite, with a part they call the pars hyoideus connecting the anterior epiglottis to the hyoid, and a part they call the pars lingualis connecting the epiglottis to the root of the tongue (they claim that this attachment to the tongue was previously unreported). Another quote from the paper follows:


  It is suggested that the pars lingualis plays an
  important role in active mechanisms such as upward
  movement of the epiglottis during phonation associated
  with tension of the lingual muscle and supraglottic airway
  patency during respiration.


To get some idea of how tension in a ligament anterior to the epiglottis can cause posterior movement (i.e. closure of the epiglottic 'flap'), I found this 1979 paper useful. They used cinefluorography to investigate the biomechanics of the epiglottis. The following lengthy quote should help to clarify the proposed model:


  The radiographic observations indicate that
  at the peak of the deglutitional excursion of the
  hyoid bone and the larynx the following circumstances
  prevail:
  
  (1) Forward translation of the hyoid bone in
  conjunction with extreme thyroid-hyoid approximation
  causes extreme elongation and
  tension on the hyo-epiglottic ligament at its attachment
  to the margins of the epiglottic
  tubercle.
  
  (2) The compression of the median thyrohyoid
  fold, secondary to extreme thyroidhyoid
  approximation, tends to make the epiglottic
  tubercle bulge backward transversely
  as well as vertically.
  
  The principle of the mechanism of folding
  deduced from these observations is demonstrated
  in a working model that can be set up
  in a few seconds. All that is needed is a rectangular
  piece of fairly stiff paper. The piece
  of paper represents the complete epiglottic
  cartilage (epiglottis and tubercle). A $\Lambda$-shaped
  crease is applied at the center (Fig. 4) to simulate
  the groove on the back of the cartilage,
  between the epiglottic tubercle below and the
  epiglottis proper above. One next applies stress to the model from the opposite side, at
  the site of attachment of the hyo-epiglottic
  ligament. This is done by grasping the paper
  lightly at the edges, below the level of the
  crease, whereupon the part above the crease,
  representing the epiglottis, immediately begins
  to fold down.
  Each limb of the crease functions as the
  directrix of a cone; the apex of the crease is
  the vertex of the cone. When the paper is
  gripped the directrices rotate towards each
  other and pull the attached paper after them,
  giving it a conical shape.


I've attached the figure 4 from the paper to illustrate better (I'm not sure if that's allowed on SE; please delete it if that's appropriate):

$\hskip 2 in$

The authors of this paper and this paper argue that the model just given does not fully explain the motion of the epiglottis during swallowing. The latter paper describes the existence of distinct lateral hyoepiglottic ligaments attaching to the ends of the greater horns of the hyoid bone. These, they argue, transmit the biomechanical forces generated by the movements of the hyoid and thyroid cartilage to the epiglottis to cause downfolding, although their full argument is too long to reproduce here.

Hopefully all of that has shed a little light on the possible function of this ligament. It seems to be quite complex, but there appears to be a relative consensus that the transmission of tension via this ligament is important in causing the folding down of the epiglottis during swallowing.