How do the Japanese actually pronounce words such as マッハ (Mahha - Mach) or シャッフル (Shaffuru - Shuffle)? I know gemination is a feature of Japanese phonology, but I've only managed to articulate such clusters as /pp/, /kk/, /tt/, /ss/. I'm not sure about /hh/ or /ff/ (?). Any suggestions?
Just start the initial sound of the kana (ハ or フ) early and hold it. It's easier to show this with romaji:
"mahhha" (extended huffing sound in the middle)
"shafffuru" (keep blowing air through your pursed lips for a little longer than usual after the シャ)
It's the same as with words like レッスン where you extends the "ssss" sound.
EDIT: And for more fun, there is an Italian-style rolled "r" in Japanese (for speakers able to do it) in Italian loanwords like タリアテッレ. Speakers unable to make the sound may just say it as タリアテルレ or some such.
ッ/っ creates a glottal stop. If you want to hear a glottal stop, listen to a Scottish person say "butter".(bu(pause)er)
Normally, when one is created in Japanese it doubles the nearest hard constanant, making it easier to pronounce. As in 日本 にっぽん Nip(pause)pon).
In words with soft constnants "ー" is often used, but that lengthens the sound. Like in セーフ. (se-e-fu)
In シャッフル it is easier to pronounce if a gap if left. So it does the same as in にっぽん but without doubling any constanants.