ouch i just did these and they really do make your legs screamWork on just the straddle lever itself. Coach Sommer trains it like this:
test your max hold and divide it in half. do 60 total seconds of volume. if your max is 10s, you would use 12 repetitions of 5s with some rest period in between, so you are not performing them one right after another.
sit in a straddle. put your hands in between your crotch and practice holding your heels off the floor incrementally. 5-10-15-30. It'll cramp your quads and make your hip flexors scream. then do the same but lift your legs up and down. you can hold, then do leg lifts or vice versa.