When gripping the 1911 with the thumbs foreward grip, your thumb can almost automatically ride on the thumbsafety.
The rason is, that when carrying cocked and locked, you position your thumb on the safety and when ready to shoot just need to press down.
Same thing the other way round. When you fired a shot to prevent accidental discharge, you just need to move your thumb a bit down, under the safety and engage it.
Also it prevents accidential engegement of the safety.
It is more comfortable doing this with the long safetylevers, but also works fine with the original ones.
Pressing your thumb agains the slide, is not such a good idea. For one, it would potentially slow down the slide and cause a malfunction and second the possibillity of injuring yourself is high.