Desire Play is the Devotional Sex word for playing with his penis
Desire is the Devotional Sex word for penis, and Play is the Devotional Sex word for touching / masturbating.
Hence Desire Play is him masturbating himself, or her playing with him using her hands.
Desire Play is an active sexual activity which takes the Knight
to a very aroused state. So less active and less stimulating activities
such as a quick touch or gentle holding of his Desire don't count as
Desire Play.
If a Princess wishes to watch her Knight play with himself she can use the Display command.
Desire Play is just one of the new phrases used on this site for talking about sex. The other new words and phrases are presented in Chapter 1 here.
The Vanilla Sex view of playing with a man's penis is discussed in Chapter 2 here.
Devotional Sex takes a more Tantric view, and Desire Play within Devotional Sex is discussed in Chapter 4 here.
A
special time within Devotional Sex is when a Princess cuddles her
Knight while gently playing with or just holding his erection. This is
called a Devotional Cuddle. This
does not count as Desire Play as any playing with his Desire is just a
gentle background activity. But of course a Devotional Cuddle can turn
into Desire Play if a Princess decides to become more active.