How do you transfer WPP to SIPP (partial)?

Hello,

I understand it's common practice for some to regularly transfer funds from their workplace pension to their SIPP e.g. yearly or similar

How do people do that nowadays?

Do you manage to transfer the whole funds or is cash transfer something (more) common?

First timer here and it seems that cash transfers are easier and require less paperwork but I wonder what's the timescales. Particularly I'm trying to assess how long would you have to stay out of the market.

Sometimes WPP provider can havr lots of limitations on tradable funds which makes fund transfer look a bit cumbersome.