If you had to ask me what I currently think is the grimmest exercise to do which is also awesome - this would likely top my list.
Rear Foot Elevated Split Squats (aka Bulgarian Split Squats) are pretty effective / brutal / will make you want to send me hate mail.
However, a lot of people do the following with a standard version:
1) Rush them and use the Stretch Shortening Cycle to bounce out the bottom of this movement, so not working the muscles as much as they could be doing.
2) Don't use enough range of motion - either to make this exercise easier to do or so they can just go for a heavier weight. Using more weight is great but don't cheat your muscles out of a decent range of motion if they can handle it.
Slow eccentric is basically a fancy way of saying lower slowly. You should look to take 4 - 6 seconds to fully lower down, going as far as is comfortable whilst maintaining a good lower back position (i.e. don't extend through the lower back and make it cranky).
• Stand tall with abdominals braced and either use bodyweight or hold a dumbbell in each hand, arms by side. Place back foot on a bench or raised surface.
• Foot can either be on toes or placed flat on edge of bench.
• Keep abdominals braced tightly and torso in an upright or slightly forward flexed position. This is the start position.
• Bend the front knee whilst lowering hips down towards the floor. Continue lowering for 4 - 6 seconds until front knee is approximately 90 degrees. Do not let back knee touch the floor.
• Press front foot into floor and squeeze glutes to raise hips straight up to return to start position.
• Repeat all reps on one leg before repeating on opposite side.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!