To play an effective defence, a partnership must communicate as much information as possible during play.
A discard is the card you play when you can't follow the suit led. It should indicate the suit preferred, so that it can be led at the next opportunity by your partner.
A partnership agrees which discard system they wish to use and this is communicated to opponents on request.
When defending, if the first discard is an odd card, it shows interest in that suit. If the first discard is even it may show suit preference - a high card asks for the higher ranking suit, a low card asks for the lower ranking suit.
Dodds uses odd or even discards to indicate a suit preference. An even discard shows preference for the suit discarded. An odd discard shown preference for the other suit of the same colour as the discard.
2H - I prefer hearts
3H - I prefer diamonds
3C - I prefer spades
When using HELD, a high discard encourages leading the discarded suit.
A low discard discourages a lead in that suit and expects a lead in a suit other than trumps.
For example, after a trump lead in a Heart contract, the discard of 9S asks for a lead in Spades. A 2S discard asks for a lead in Clubs or Diamonds.
The HELD system can be reversed with a Low discard to Encourage and a High discard to Discourage.
This has the advantage of not discarding higher cards in the preferred suit.
When using McKenney, the discard excludes a preference for trumps and the suit discarded.
If a low card is discarded then the lower of the remaining suits is preferred.
A high discard shows a preference for the higher suit.
A 5 or less is a low card.
A 7, 8 or 9 is high.
6 is a problem.
For example, if a trump is led in a Heart contract, a discard of a S shows a preference for a C or D lead. Discarding a low card such as 2S shows a preference for the lower of the remaining suits - Clubs.