Slash ecchi might nod toward a flirtation between characters of the same gender, and hetero ecchi might manifest in silly plot-twists which require a character to strip down.
This kink can be quite similar to cuckolding, but it often has a 1970s swinger twist.
A more modern and progressive example is all the care taken in animating Yuri!!! Chris Evans famously pushed for an example of fan service while filming Captain America: Winter Soldier when he asked the Russo brothers to let him film a stunt shirtless.