To me, engagement in play means, really getting the child to be present and in the room with you. I have yet to find a better way to connect with a child than play. Play is the way children communicate. It is the way they connect and engage with their inner and outer world.
Rumi, the ancient Afghan poet once wrote, “When your with children, talk about toys. From playthings, little by little they reach into deeper wisdom and clarity. Gradually they lose interest in their toys”.
When working with a young child on the spectrum, specifically a largely non-verbal or withdrawn child, reciprocal play cannot be beat. Stanley Greenspan and Serena Wieder’s DIR-Floortime model describe this approach beautifully in their book, “Engaging Autism: Using the Floortime Approach to Help Children Relate, Communicate, and Think”.
Reciprocal play focuses on engaging the child in child-directed play. This type of play does something Greenspan and Wieder call “building circles of communication”. Essentially, in doing this the caregiver is keeping the child present in back-and-forth exchange for longer periods of time and in the room (not withdrawn).
One fundamental component of Floortime is “obstructive play” – literally pushing your way into your child’s solitary play (putting your car on the tracks of his train set just to force interaction).
spectrumshare.com 
Very good article – however, please give examples of the types of play
In the child’s typical play area, with the child’s selected toys, get down on the floor and play as he/she does. However, do not wait to be invited into play (it often won’t happen). Inject yourself into the child’s play scenario by taking on a character (ex: Lego pirate, truck that wont get out of the way until the child says/signals “move”, etc.) and begin to directly interact with the child’s toy (train set, pirate ship) in a way that will get him/her interested in you and what you are doing. This takes silliness and creativity on your part. Once you get the child to involve you in his/her play, it is your goal to remain involved as long as possible without “losing the child”. Your direct involvement/engagement with the child in play is vital in the child’s social-emotional development and language development.