When off-handing (default key is F) and placing a block in the same tick the item gets placed for a very short time on the client which makes "ghost blocking" or "ghost lily pads" possible.

Despite this the item still exists in the inventory which means this can be done infinitely (albeit it very hard to do).

After pressing the offhand key once and placing the first block, in order to keep going you need to from then on press the offhand key twice in order for it to work.

This probably relates to these bug reports:

MC-154881

MC-240720

MC-277905