"Now for the warm up for the test."
"I want you to give me 50 really really slow strokes while you admire my long legs."
[10 seconds rest]
"Then give me 50 more strokes, a bit faster than the last 50 but still slow."
[10 seconds rest]
"Then end the warm up with 50 medium paced strokes, take 30 seconds rest and continue on to the final test."