Along with the statement:
(Note since we weren't there to hear what the coach said in it's full context, that may inject some confusion that can affect the replies)I had long discussion with my coach today about this and he had strong opinion that I definitely should practice bowling with sneakers.
His argument was that it forces me drop my waist level during 4th step which leads to strong release position where it can be easily pull down instead of push. And from that position I can more easily get more speed and accuracy to ball.
IMO No the dropping of the waist in of itself doesn't generate more speed or accuracy.
It's the being in the lower (stronger) position, that allows you to transfer the power/speed you already generated with your approach and swing more efficiently and allows you to be more accurate.
Which appears to be what your coach is wanting by where you said "...which leads to strong release position ....."
The first question then would be how do you achieve a lower waist position?
The basic answer is to finish with good knee bend and your trailing leg behind you to get the hips low. (Making sure to post the shot so your not falling off.) this will give you a more consistent, accurate and a stronger release.
The next question is How would wearing sneakers (and doing a plant and shoot method) going to lower the waist and generate speed?
You said your coaches argument was "that it forces me drop my waist level".
By planting and not sliding, your bringing your body basically to a dead stop. Your leg will then act like a shock absorber, the knee will bend some and because of this your body will get a little lower.
But by stopping that way you've eliminated most of the momentum (speed) you built up during your approach.
So How do you get the speed (momentum) back?
Most likely it's going to be by the player muscling the ball, Pulling it down in the swing.
In your post that is mentioned by your coach "where it can be easily pull down instead of push." + the trailer example.
Pretty much everyone will tell you don't pull (muscle) a ball down, that can lead to several problems. You want a loose swing motion and when your ball is at the top of the back swing and it starts forward you want the ball feeling like it's falling into the forward swing. You could then accelerate your forward swing as you go into the finish.
Earlier I asked:
"Question when he said to "practice" bowling with sneakers, Did he just mean to use sneakers as a training aid during practice to get you use to dropping your waist level ? or to use them all the time even in league?"
Yes in bowling there are many thing's that there is no right or wrong way to do it, you do what works for you.I think that only thing which is agreed by all the coaches who with I have discussed is that there is no anything so good or bad on bowling what you should always or never to use.
Point is to give bowler tools where he can pickup which works for him.
But That doesn't actually answer the question asked.
You see there are thing's that coaches sometimes tell you do in "PRACTICE", because they are just meant to give you a feeling of something, a certain position etc. or to help you develop a certain muscle memory. But they don't mean for you to necessarily do it in actual competition.
The question was how did your coach mean it (to Use sneakers)?
Use the sneakers only during practice as a training aid of some kind? To get the feeling of being lower etc.
Or does he want you to use them all the time, change from a slide and become a plant and shoot bowler?
Because if you go to a plant and shoot method, that will affect the replies given. Such as the talk of the flat spot etc.