Hey dude, you do yourself no favour with your haughty reply to flying apple's observation about your " strange flight levels ".....a hint of arrogance at your current ability to fly modern shiny jets in PRC. Secondly, your query betray a complete lack of understanding of VNAV vis-a-vis basic vertical profile management. Some wonder how you manage to garner a job flying this shiny new jet around the PRC. Now you last post clearly takes the cake.
Now I don't fly the B738 but I have previous experience on the B767. When you set FL148 on the MCP without pushing ALT INVT, the FMC cruise altitude remained at FL256 on VNAV cruise page. If you pressed VNAV after being at ALT HOLD, VNAV descent would not occur because there was no defined active descent path from current airplane altitude to the FMC VNAV cruise page. Then by engaging ALT INVT you had either resetted the FMC cruise altitude on the VNAV cruise page to FL148 or activated the VNAV DES function. When FL148 was propagated to the FMC cruise altitude on the VNAV cruise page, there was now a defined cruise descent function which allowed VNAV to engage. If you had passed TD, then the VNAV DES would be activated. In both cases, the AFDS would descend the aircraft; in the first case it would be a cruise descent and in the second instance it would be descent at idle thrust at VNAV SPD trying to achieve VNAV PTH.
I hope you and JT finds this helpful!
I fly airbus products now; so my recollection of Boeing VNAV operation may be faulty.
I understand haejungkuk's lament because I have seen a quite a number of newbies who bought rapid type rating training which never prepared them for flying the shiny new jets at all. They then had the miraculous 500 or 1000 flying hours in their logbooks when they show up in the Orient for plum jobs. Your question at the start of this thread is one of the many we have come across. The interesting thing is that the some " knowledgeable " replies to the query will certainly raise eyebrows, if not scare a lot of people.