Have a 'Polo'

The trainer was giving last minute instructions to the jockey at the starting gate when he appeared to put something in the horses mouth, just as a Steward was passing.

"What was that?" demanded the Steward.

"Only a Polo" said the trainer and gave one to the Steward and took one himself.

The Steward appeared satisfied and went about his business.

The trainer went on with his instructions: "listen, you're on an absolute certainty today. Just hold the rail until the home straight. The only thing that could pass you then

Spoiler:

would be either me or the Steward.