A multi-contraction drop set requires some specific equipment: light resistance bands and Mark Bell's slingshot. I find this to be the best option but I'll provide other ones for those who don't have these tools.
First do 5 reps with added band resistance. Then remove the bands and immediately do 5 more reps. Then put on the Slingshot and pump out as many reps as you can. Do 3 sets.
If you don't have access to a Slingshot and bands here are two options you can use:
Five reps bench press, then as many top-half bench press reps as possible (going down until elbows are just above 90 degrees). Then hold the top position, squeezing the pecs for 15-30 seconds.
Take 10 seconds between each step.